Where the sign once hung

What sign used to hang here?

it was one of their educational signs. It said "This is a wall".

There was too much education in that though, so they hired a new educational sign-writer. The one who comes up with "What if a banana could eat a monkey?"

What's it going to say now? What if walls could climb spiders?

Perhaps they could take the Magritte approach and hang up a picture of a wall, labelled 'This is not a wall'.