How did Underdog get his powers?

Answer 1

Simon tells the dog that it's going to hurt a lot, which causes the dog to sneeze into Simon's mouth, then jump out of his hands. As he runs around, the dog causes severe damage to the lab and knocks a shelf containing chemicals onto himself, and it starts a fire. The chemicals change his body around and make him able to run out of the lab by smashing his way through the wall, but after setting the lab on fire.