A Proton, a neutron and Helium walk into a bar...

A proton, a neutron and Helium walk into a bar and order three beers. The bartender appears with three beers in hand and asks the proton, “Are you sure you are above 21?” The proton replies, “I’m positive.” The bartender then gives the proton his beer. He then says to neutron while giving him the second beer, “For you, no charge.” He then proceeded to throw the last beer into Heliem’s face. Helium didn’t react.