A Country Yokel

A country yokel and a professor were in a train,
and as it was a long journey they eventually got to talking,
"Everytime you miss a riddle you give me a pound,
and everytime I miss one I give you a pound"
said the professor. When they had run out of the
usual things to talk about.
"Ah!, but you're better educated than me, so
I'll give you 50 pences (0,5 pound) and you can
give me a quid (1 pound)" suggested the yokel.
The professor agreed and the yokel made up the first riddle :
"When has three leys walking and two legs flying?"
The professor didn't know, so he gave" the yokel a pound.
The yokel didn't know either, so he gave the professor 50 pences.