Mirror Wing
A symmetrical shape is one where every mark has a matching mark across the line.
A grid with a mirror line down the middle. You draw on one side of the line only — and every square you fill appears at once on the other side, the same distance from the line. There is no way to draw something that is not symmetrical, because the mirror will not let you.
First you just draw and watch. Then a whole butterfly is shown and you have to make it by drawing only its left half. Then a second mirror line arrives, across the middle as well as down it, and one square you fill turns into four.
Nothing can go wrong and nothing is lost: filling a square you did not mean to fill just means filling it again to take it away, and "Clear" empties the grid in one press.