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.