If you don't understand why this guy (here, too)is absolutely hilarious, it's time to go back to school for some science education. (via Geekdad)
I remember telling my old boss a joke:
An ion walks into a bar. The bartender asks why he's so sad. The ion says, "I lost an electron." The bartender says, "Are you sure?" The ion says, "I'm positive."*
My boss agreed it was a good joke and lamented that so few people would get it.
*An ion is a particle that has a positive charge because it has too few electrons, which have a negative charge, to balance the positively-charged protons in the nucleus.