Three closed doors are numbered 1, 2, and 3.

Behind one door is the nice behind of Vanna White who sits on a stool smilling. You like her.

Behind the other two doors are bags of Jim Dandy dog food. You don't have a dog.

You can have what is behind the door you select...so select.

Nice choice. But before openning the door that you have selected, Monty Hall opens one of the other doors and shows you a sack of Jim Dandy dog food and asks you if you would now like to change your first selection.

You should.

Why?