Echo

the room is invisible. ping it.

tap to ping — walls sketch themselves out of echoes

Sound bounces off a surface and returns after a delay set by distance alone — time-of-flight. Enough pings, each timed and located, are enough to sketch a room's walls with no light involved, the same principle bats, dolphins, and submarine sonar use to see in the dark.

Concepts: sound, space.

An interactive you play with one thumb, on Wundr.