Psychology Project----what's a confounding variable???

[quizbowlgeek]

We did an experiment where a person said a tongue twister 5x as fast as they could and we timed it. then they dribbled a basketball while saying the tongue twister 5x with their right hand and next with their left hand. Now we need to find what a possible confounding variable that was in the experiment. Please write down any suggestions whatsoever!!
our hypothesis was lefties would be faster than rightys and that rightys would be faster while dribbling with their left hand.