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 fluidisation, 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 inbetween 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.