Two hydrogen atoms walk into a bar...

Two hydrogen atoms walk into a bar. One says, "I think I've lost an electron!" The other says, "Are you sure?" The first says, "Yes, I'm positive."

This is a physics joke, and also a pun. The critical element of the joke is that atoms are normally electrically neutral, however if an atom were to lose an electron it would become electrically positive. The pun is that people say they are positive to mean that the are entirely sure that something is true, and to also mean in an electrically positively charged state.