Cut one slot in the lid of a box, just wide enough for a bottle cap.
Show your child how a cap goes in and disappears. Then hand them the caps and step back.
When a cap will not fit because they are holding it flat, resist turning it for them. Say what you see instead: 'it is too wide that way.' Let them find the rotation themselves — that discovery is the whole activity.
Rotating an object to match an opening is spatial problem-solving with instant, honest feedback: it either drops through or it does not. The child adjusts against reality rather than against a parent's approval.