Friend says we should pay up but I don't see how we owe her!?

A couple days ago some friends and I were goofing off at the park in our neighborhood. One of them challenged me and another girl to a race. She said she'd bet us five bucks each that we couldn't beat her. So we all lined up and at the cue took off for a tree that was supposed to be the finishing point. We got about half-way there before the two of us found ourselves being jerked off our feet and onto our butts! Somehow, one of our friends had managed to tie separate ends of what appeared to be a discarded jump rope (with the handles missing) to each of our belt loops and during the race we'd both run past a lamp post on opposite sides. We all had a good laugh about it. We had to admit, it's a prank you don't see every day. And I still have no idea how they managed to tie us without us noticing. But here's the thing. Our friend actually expects us to pay up, since the bet was that we couldn't beat her and, obviously, we didn't. I don't see how it's fair to expect someone to pay up on a bet they only made cause they didn't realize the play on words involved. What do you think?