Bury

you do not choose what arrives. only where it goes.

drop falling gems into four columns by colour; mix two colours and the column locks, then clears for nothing

A gem arrives and you choose only where it lands, atop whatever is already there. Four of one colour clears the column; a second colour crosses it out, locking it three seconds before it empties for nothing.

Concepts: irreversibility, constraints, planning.

An interactive you play with one thumb, on Wundr.