O.K. I'm basicly just summarizing what every one else is saying, but here it is:
tamagotchi's gain weight on certain stages. The bare minimum weight for each is this:
baby=5 pounds
todler=10 pounds
teen=20 pounds
adult=30 pounds
some characters are different but most follow that.
Now to play a game you have to be at least 3 pounds overweight for any of these stages to play a game. So babys have to be at least 8 pounds, todlers 13, teens 23, and adults 33. I personally dont' suggest ever feeding your tama when it's not hungry or giving it a snack when it's not unhappy, as this can make it sick and make it not be hungry/unhappy for a long period of time afterwards (and that's boring!). So if I were you I would just wait for it to want to eat something, then feed it, then play some more games.
hope this helps.