A man slow witted man walked into a patent office. He walked up to the patent officer and said, 'Hey, I've got a new idea for a mouse trap.' (Draws a box on the blackboard.)

'Here's the box.' (Draws a hole in the box.) 'Here's the hole.' (Draws a circle in the bottom of the hole) 'Here's the cheese.' (Draws a line across the hole in the box.) 'Here's the blade. The mouse sticks his head in the hole to get the cheese, the blade drops on his neck and kills him.'
The patent officer looks at the diagram. He understands that the man is a little slow so he wants to be kind. He explains to the man that he does not think the design is ready to be patented yet.

He tells the man, 'Please, work on it some more. Perhaps I will be able to patent it another time.' The slow witted man says thank you and leaves the office.

One week later the slow witted man shows up again. (Draw the exact same example on the board in exactly the same way.) The slow witted man says, 'This is the box, this is the hole, this is the cheese and this is the wire. The mouse sticks his head in the hole to get the cheese, the wire wraps around his neck and kills him.' The patent officer, still trying to be kind, makes the same excuse as before. The slow witted man leaves.

One week later the slow witted man returns. He approaches the same patent officer and says the exact same things

 
'Here's the box. Here's the hole. (This time he draws a zig-zag line across the hole and he does not draw a circle for the cheese.) After completing the zig-zag line, the slow witted man proclaims, 'and here's the saw blade.'

 
The patent officer notices the design and the fact that that there is no cheese. He asks the slow witted man, 'Where's the cheese.' 'Ah-ha,' says the slow witted man.

'That's the point. The mouse sticks his head in the hole and says,' 'Where did you put the cheese.