A man walks into a pharmacy and asks for a pack of condoms. As soon as he has paid for them, he starts laughing and walks out.

The next day, the same performance, with the man walking out laughing fit to bust. The pharmacist thinks this odd and asks his assistant that, if the man returns, to follow him.

Sure enough, he comes into the store the next day, repeating his actions once more. The assistant duly follows. Half an hour later, he returns. So did you follow him? I did. And…where did he go? Over to your house!