One day three doctors went to a convention together, on the way back, they noticed the car was slowing down. They got out and looked at the tires. The first doctor said "I think its flat."

The second felt it and said "It feels flat."

The third stares at it and says "It looks flat."

All of them, without taking their eyes off the wheel, shook their heads and said in unison, "We better run some tests."