A word on a box moves along a belt at the spelling factory.
On the right you see one of the three suffixes ed, er or ful.
At the top of the screen is a rule for adding that suffix.
If the rule is used for the word you see, click the tick on the right side of the rule. If it is not correct click the cross on the left. The box will then move along the belt and onto a waiting lorry. If you are wrong the lorry will drop the box. If you are right the lorry will take the box away.