A comment on another thread about people with PhDs saying dumb things reminded me of somthing a professor once told the class when I was a student. I thought of sharing it with you, in the hope that some of you also have interesting stories to share. In an organic chemistry course, the professor was discussing proton NMR, as 1H NMR is commonly called for obvious reasons. One student asked why the protons in other nuclei in the molecule would not also show up on the spectrum (*), the prof answered "because in other nuclei, there are positrons, not protons." (*) For those who don't know how NMR works: you basically induce spin flips in the nuclei, such that it is the total spin of the nucleus that is important. Most nuclei have spin 0, so you can't do NMR with them, and others resonate at completely different frequencies, so you only see one element at a time in any case.