The playground you frequented as a kid was probably blanketed in sand to help soften the impact every time you tumbled off the jungle gym. But did you know that something magical happens when you pump a pile of sand full of air? It suddenly enters a liquid-like state.
The effect is known as fluidization, and it will happen with almost any fine powder or granular material when you introduce a constant flow of air from underneath. As the air makes it way towards the surface and through the particles, it gets in-between each granule and reduces the friction that normally makes sand behave more like a solid. The resulting mixture moves and flows like water, even allowing certain objects to float or sink.
In theory this approach could be used to surround and protect your home with a moat full of switchable quicksand. You’d be easily able to walk across it, but when an intruder is detected, switching on a compressor would cause them to sink like a stone.