Well, this one requires a bit of esoteric knowledge of art, but I don't think it's too obscure. If you don't know what the French phrase means, it means "this is not a pipe". If you still don't get the joke, I suppose I can give you uncultured ignoramuses a SPOILER.
And don't challenge me on calling you uncultured ignoramuses. I've got no basis for it. If you try to prove me wrong, you will succeed. Congratulations, you win. I bow down to your superior ability to be improperly insulted.