A neutron walks into a pub, pulls up a stool at the bar, and calls out, "Barkeep! A tankard of your finest ale, please sir!" The barkeep is slightly taken aback, as he's never seen a subatomic particle in his bar before. But he recovers quickly, and pulls the neutron a pint of ale. The neutron doesn't get out much -- he's usually kept at home by a strong force -- and he's really smiling as he enjoys his brew. He finishes it quickly, then stands up and asks the barkeep, "How much do I owe you?" The bartender looks at the neutron, smiles, and says, "For you sir, no charge."
An hour or so later, a neutrino walks in the same place, pulls up a stool, and asks in a high voice, "Barkeep, may I please have a pint of ale?" The barkeep now feels himself an expert at serving subatomic particles, and quickly pulls the neutrino's beer. The neutrino picks up the glass, takes one sip, and falls over on the floor, dead drunk. The barkeep looks over the edge of the bar at the neutrino, shakes his head, and mutters, "Lightweight."
[Q.v. Physics jokes II]