Down

tap a wall. now it is the floor.

a glass box of sand and three blocks: tap any wall and it becomes the floor, and everything loose pours toward it

Down isn't a property of the world, it's a direction the world happens to agree on, and here you overrule it. Tap any wall and gravity turns to face it: loose sand and blocks fall toward whichever wall you chose.

Concepts: gravity, physics.

An interactive you play with one thumb, on Wundr.