"When in doubt, think." joke

A friend of mine and his boss went boating a few weeks ago, and they
decided to go look at some of the well-known landmarks in the
Chesapeake.
Well, they were getting near one of them and the boat ran aground.
Unable to back it out, they finally had to get out and push. While
they were doing this, an employee at the site came out and watched,
and the hapless boaters explained that they'd been looking at the
landmark when they got stuck. This exchange followed:
Employee: Sir, do you know what this is?
Boater: A lighthouse.
Employee: Why do you think it's here?