Aerogel is a solid material obtained by replacing all the liquid in a gel with air while still retaining the original nanoscale framework. In simple terms, it is "dry and hard foam smoke". 
To say that it is lighter than air is actually a clever play on words - in normal circumstances, the pores of the aerogel are filled with ordinary air, with a density typically ranging from 3 to 50 milligrams per cubic centimeter. It may seem heavier than air (1.29 milligrams per cubic centimeter), but if the internal air is evacuated to a vacuum, the lightest all-carbon aerogel can achieve a density of 0.16 milligrams per cubic centimeter, which is indeed lighter than the same volume of air. It feels almost weightless when held in your hand, and it won't even bend the flower stems when placed on the downy fuzz of a dandelion. 
This seemingly fragile thing actually managed to make its way from the aerospace laboratory all the way into the lives of ordinary people through sheer strength. 
The most remarkable feature of aerogel is its excellent heat insulation property. It can be said that it is one of the solid materials with the strongest heat insulation capacity that humans can produce in large quantities at present, and this statement is by no means exaggerated. 
Its structure is like stacking countless nanoscale soap bubbles together. Over 90% of the space is filled with air, while the diameter of the solid framework is only a few nanometers, tens of thousands of times thinner than a hair. Heat transfer occurs either through molecular collisions, or through solid conduction, or through thermal radiation. However, for aerogel, all these three paths are blocked: the air molecules are trapped in tiny holes of a few nanometers, making it impossible for them to collide and transfer heat; the solid framework is too thin, and the path for heat conduction is extremely long, so that the heat dissipates after just two steps; if some shading particles such as titanium dioxide are added inside, even thermal radiation cannot pass through. 
The actual test data is presented here: At a room temperature of 25℃, the thermal conductivity of silica aerogel is only 0.012 - 0.016 W/(m·K), which is half lower than that of still air (0.023 W/(m·K)), one-third of the common insulation material rock wool, one-fourth of the filling material down feathers in a down jacket, and one-twentieth of an ordinary red brick. 
Let's take the most intuitive example: A 1-centimeter-thick aerogel sheet has a heat insulation effect equivalent to 20-30 ordinary glass sheets, or 10 centimeters thick rock wool board. If you hold it in your hand and place it under a 1000°C blowtorch, your hand will only feel a slight warmth, not even hot enough to be considered scalding. If you put a slice of grilled beef between the blowtorch and the aerogel, after burning it for half an hour, the beef can remain at a medium-rare state with no burnt spots on the surface. A previous outdoor blogger conducted a test. In winter, wearing an inner jacket filled with 5 millimeters of aerogel and a windproof outer jacket, standing in a snowfield at -20°C for two hours, the back of the body could maintain a temperature above 30°C. To achieve the same effect with an ordinary down jacket, the down filling would need to be over 200 grams. 
Previously, the rockets used for space launches and the Mars probes all had to be coated with aerogel insulation layers. Otherwise, when returning to the atmosphere at temperatures of several thousand degrees Celsius, the instruments inside would have melted long ago. The NASA "Stardust" probe even used aerogel as a "catcher" to capture cosmic dust traveling at several thousand kilometers per hour in space - if these dust particles hit ordinary materials, they would simply break apart. But when they hit the aerogel, it was like being caught in a layer upon layer of cushioning nets, gradually slowing down the speed and maintaining the original structural integrity. This feat could not be achieved with any other material. 
Ordinary silica aerogels are indeed brittle and can break into pieces when dropped on the ground. However, the modified aerogel materials now have much greater toughness than rubber. The aerogel mats used in industry can be bent at will, rolled into tubes, and even cannot be broken when stepped on. Some manufacturers have produced aerogel composite materials that can withstand pressures 1000 times greater than their own weight, and are even stronger than plastic of the same weight. 
It also has an extremely impressive feature: resistance to extreme temperatures. The usage temperature range of silica aerogel is from -200℃ to 1200℃. It won't crack when frozen in liquid nitrogen and won't melt when burned in a fire. Its stability is stronger than that of most materials. Currently, the inner layers of many LNG (liquefied natural gas) transport ships' tanks are coated with aerogel insulation layers. The temperature of liquefied natural gas is -162℃. If using ordinary insulation materials, it is prone to shrinkage and cracking over time. However, aerogel maintains its performance for over 20 years without any decline. Moreover, it occupies half the space compared to traditional insulation materials, making it even more cost-effective. 
Even some people use aerogel as the buffer layer for bulletproof vests - when a bullet strikes, the nanoscale framework of the aerogel instantly absorbs the impact force, dispersing the concentrated energy across the entire surface. It is 30% lighter than traditional Kevlar material and can enhance the bulletproof effect by 20%. Now, many special forces units in various countries have begun to trial it. 
However, it is not without drawbacks. The most troublesome issue is powder shedding. The surface of ordinary silica aerogels contains many nanometer-sized particles, which can cause itching when rubbed against the skin. Therefore, the current civilian aerogel materials are all treated with a sealing layer. They can either be pressed together with non-woven fabric or filled with polymers to form composite materials, thus solving the problem of powder shedding. However, the cost will be slightly higher as a result. 
Twenty years ago, aerogel was only used in the aerospace field. One kilogram cost tens of thousands of dollars. Now, with the maturity of mass production technology, the price of industrial-grade aerogel has dropped to several hundred dollars per kilogram, and that of civilian-grade aerogel is only a few hundred dollars. It has quietly entered every corner of people's lives. 
The most common ones are thermal insulation products. Nowadays, many outdoor brands' windproof jackets and hiking boots have added an aerogel layer. The thickness is only 2-3 millimeters. Even when worn in a temperature of minus 30℃, your feet won't get cold. It is half as light as thick wool boots. Some people also make aerogel thermal insulation patches, each only as thick as a coin. They can keep heating for 8 hours. They are two-thirds thinner than ordinary heating pads and can be stuck on clothes without being noticeable. The renovation of old residential areas in Northeast China now also uses aerogel for exterior wall insulation. Just applying 1 centimeter thick layer can achieve the insulation effect of the previous 5 centimeter thick polystyrene board. Moreover, it won't catch fire and won't age or fall off after being used for 30-50 years. As a result, the annual heating cost can be saved by more than 30%. 
The new energy vehicle industry is also rushing to use aerogel at present. The biggest concern for electric vehicles is battery thermal runaway. Once a single cell catches fire, the temperature can instantly rise to 800℃, which can easily ignite the entire battery pack. Now many car manufacturers are attaching 2-millimeter-thick aerogel insulation sheets between the cells. Even if a single cell catches fire, the temperature will not spread to the adjacent cells, allowing drivers at least 5 minutes to escape. This is something that no other insulation material can achieve. Previously, a manufacturer conducted a test. Using a blowtorch to burn the battery cells with aerogel attached, after 10 minutes of burning, the temperature of the adjacent cells did not exceed 60℃, and the outer shell did not even deform. 
Nowadays, even many high-end thermos cups and cooler boxes for camping also incorporate aerogel in their inner layers. If the vacuum layer of an ordinary thermos cup is damaged, its insulation effect is lost. However, a thermos cup with aerogel added can maintain its insulation effect even without the vacuum layer, and it can still be used even if it is dropped and deformed. For camping use, the aerogel cooler box is only half the size of a regular thermos box. It can keep ice cold for 72 hours, which is twice as long as the regular model. Carrying it for hiking is completely effortless. 
Even the construction industry is using it. Nowadays, many art galleries and museums use aerogel as the glass interlayer for their display cases. This not only provides insulation but also blocks ultraviolet rays. Unlike ordinary interlayer glass, it doesn't fog up and affect viewing. The cultural relics placed inside won't have significant temperature changes even if exposed to the sun, and the aging process can be slowed down by half. 
Of course, nowadays there are also many so-called "intelligent product" items on the market that are actually scams under the guise of "foam polymer". When purchasing, one must be vigilant. 
The most common ones are the air-sol胶 insulation underwear that cost around 40 yuan. The actual cost of the air-sol胶 insulation material is not low. The cost of a garment filled with qualified air-sol胶 insulation material is at least 100 yuan or more. The so-called air-sol胶 underwear priced at 40 yuan is either just adding a little air-sol胶 powder to the collar and cuffs， or not adding any at all. It's purely a concept炒作. When you wear it， it's no different from an ordinary autumn dress. There are also those air-sol胶 belts and pillows that claim to be "self-heating". They are basically all deceptive. Air-sol胶 itself does not generate heat； it just has good insulation properties and can lock in your own body temperature. Claims of self-heating are all lies. 
Some people even claim that aerogel can be used to make masks and can remove formaldehyde. This is just nonsense. The pores of aerogel are indeed at the nanometer level, but it is not as effective as ordinary melt-blown fabric for filtering PM2.5. It has poorer air permeability. As for removing formaldehyde, aerogel itself has no adsorption function. Unless its surface is loaded with adsorption materials such as activated carbon, its formaldehyde removal effect is even worse than opening the windows for ten minutes of ventilation. 
Overall, aerogel is a functional material that excels through its structure. Its strength is derived from the physical structure. It cannot replace steel as a building material or be used as fabric for clothes. However, in specific areas such as heat insulation and shock absorption, it is a truly outstanding player. In the future, it will increasingly appear in our lives, not as an unattainable concept.