An infinite number of mathematicians walk into a bar. The first one orders a beer. The second orders half a beer. The third orders a third of a beer. The bartender bellows, "Get the hell out of here, are you trying to ruin me?"
Teacher: What's 3 minus 1?
Me: I don't know.
Teacher: How about this, you have three cakes, I take one. How many cakes do you have?
Me: Three.
Teacher: If I take one cake from your three, what do you have?
Me: Three cakes and a dead teacher.