Gravity

a box of marbles in your hand. tip it.

a box of a hundred and thirty marbles with slides, pegs, gears and two funnels: tilt the phone and they run the course, and every hard landing is a knock you feel

The phone is the box. Tip it and the marbles fall toward the real down, measured by the phone. Every marble that lands hard enough is one knock in your hand, heavier for a wall than for another marble. No motion sensor? Hold where down should be, and tap to knock the box.

Concepts: gravity, collisions, touch.

An interactive you play with one thumb, on Wundr.