Two economists were walking down the street. The first economist spotted a £20 note on the ground and stopped to pick it up. The second economist asked "why are

Two economists were walking down the street. The first economist spotted a £20 note on the ground and stopped to pick it up. The second economist asked "why are you stopping?". The first economist said "to pick up this £20 note". The second economist replied "don't be stupid, if there was a £20 note on the ground somebody would already have taken it". The first economist considered this for a moment, then nodded, and they both walked on.