Doors

Holly can't understand doors. She doesn't realise that if she wants to leave through one, she cannot be sitting in front of it at the same time. She stands at the back door and her feet get trapped when it opens, however slowly you open it; ditto the cupboard doors, except it's her head.

I always thought this was a kind of instinctive knowledge - doors. But obviously not, or we have a very stupid dog.