So I don’t get it wrong again

Here’s the physics joke I’m always trying to retell:

Professor Heisenberg is cruising along the highway when he suddenly gets pulled over. The motorcycle cop sidles up to Heisenberg’s window. Heisenberg rolls it down.

“Do you have any idea how fast you were going?” asks the cop.

“I have no idea,” Heisenberg replies. “But I know exactly where I am.”