Not necessarily - For every bet each party lays down a stake. In poker, you have to match the stake or drop out. In pontoon, the dealer matches your stake exactly. But in horse racing, the bookkeeper often stakes a lot more than the punter, due to the low odds of the punter actually winning.

In this case you've defined the condition, and the stake that you're wagering, but have left her side of the wager implied. In bets where the condition is ridiculous (bet you can't eat three live goldfish!), the bettor often only expects to win the satisfaction of seeing the other party humiliate themselves in an amusing manner...

Of course, if the bet was in all seriousness, both of you should have laid your money down into a pot (or in escrow with a third party), leaving no room for ambiguity. And make sure you shake on it!
In the case of the piano bet, at least one party could control the event. If they tried hard enough. Maybe.