Teaching by doing
Also said as Show, don't tell, Silent teaching, Level as tutorial
Building the level so the player works out a rule for themselves: first a safe place to try it, then a small test, then a real one. No text is needed.
What you work out yourself sticks, and it feels like your own cleverness. The first use has to be safe, though. If the first thing a new ability is needed for can kill you, you are testing people before you've taught them.
Say it
“Introduce the double jump in a room with no danger and one high ledge. Next room, the same ledge over a shallow pit. Only the third has a gap that kills.”
Where you’ve seen it
World 1-1 of Super Mario Bros. teaches running, jumping, blocks, mushrooms and enemies without a line of text, and almost everything later in the game is a variation on it. Portal's first test chambers introduce the portal gun through a sequence of simple rooms, each building on the last.
Not to be confused with tutorial
A tutorial explains the rule. Teaching by doing arranges the world so that the rule explains itself.