The difference between theory and experiment:

A guy was walking along the street one night, when he came upon a man–a
theoretical physicist–on his hands and knees under a street light, searching
the street. The fellow asked him what he was looking for, and the theoretician
replied, Im looking for my car keys. Being a helpful sort, the fellow
started searching, too.

After a time he asked, Are you sure you lost them here?

Of course not replied the theoretician. But at least theres light here.