Two vampire bats are hanging upside down in a cave. One of the bats feels rather hungry. ‘Let’s go and find some blood’, he suggests.
‘I don’t think you can get any blood this time of the day’, says the other bat.
‘Well, I want some blood and I want it now!’, says the first bat and prepares to take off. ‘Are you coming or what?’
‘Don’t be stupid, you’ll just waste your time’, says the second bat.
So the first bat flies anyway. After a few minutes he is back with blood smeared all over his face. ‘Where did you manage to find blood this time of the day?’, asks the second bat.
‘Well, do you see that tree out there?’, says the first bat and points at a tree outside the cave, ‘Do you see that tree?’
‘Yes’, says the second bat, ‘of course I see it’.
‘Well, I didn’t’, replies the first.