Slot

aim by the shadow, not by the thing.

drop each shape through its own hole -- by lining up the shadow, not the shape

A shape hovers above a lit lid with three holes cut in it, and its shadow lies off to one side because the light is not directly overhead. Holding the shape over a hole does nothing; landing its SHADOW in the hole is what drops it through. Two things come out of that: a shadow is really a readout of where a thing is in the plane that matters, and even with perfect aim only the shape whose own outline matches the hole will go through -- which your eye settles faster than you could argue it out.

Concepts: light, perception.

An interactive you play with one thumb, on Wundr.