How it works
A slip is only something not right yet. Everyone slips, even grown-ups.
Backspace rubs out the last letter you typed. Tap it once and it nibbles the last letter, tap again for the one before.
Go back only as far as the slip. The good letters after it get rubbed out too, so you type them again.
Tap and look, then tap and look again. Holding backspace down keeps munching and munching.