My favourite maths joke

Here is my favourite maths joke. It's really a joke about mathematicians and not about maths.

Two men are travelling in a hot air balloon. The wind suddenly freshens up and the get carried away inside a cloud. Once the wind dies down the two men are properly lost. Fortunately they see a man walking along a road below them. As they are heading for the man they call towards him "Excuse us Sir, could you please tell us where we are?"
The man stops dead in his track and looks at the two travellers, but he doesn't answer. The balloon continues to fly towards the man on the road until it is straight above him. Still no answer. The balloon flies on and, just when it is almost too far away and the two travellers had almost given up any hope of getting an answer, the man on the road shouts to them: "Dear Sirs! You are located in a basket beneath a hot air balloon."
The travellers look at each other in astonishment, until one of them says "That man was a mathematician." "How do you know that?" asks the other. "Well, there are three reasons
a) He took a very long time to come up with a response.
b) His answer was absolutely correct.
c) What he said did not help us in any way."