The organic chemist... 'cause we can explain anything :P

The analytical chemist comes running excitedly into the organic chemist's office, waving a graph taken off his latest experiment. 'Hmmm,' says the organic chemist, 'That's exactly where you'd expect to see that peak. Here's the reason (long logical explanation follows).' In the middle of it, the analytical chemist says 'Wait a minute', studies the chart for a second, and says, 'Oops, this is upside down.' He fixes it. 'Hmmm,' says the organic chemist, 'you'd expect to see a dip in exactly that position. Here's the reason...'.