Unravel

Every block on the board carries an arrow. Touch one and it slides that way until it leaves the board or runs into another piece and stops there. Clear the board before your moves run out. That is the entire rule set of Unravel, and the whole difficulty is sequence: take the pieces in the wrong order and the board locks into a shape with no solution left, with moves still on the counter and nothing to do with them. The early levels teach the grammar; the later ones want the board read before the first move. A hundred levels, all built by hand.