Rice CRISPY Treats

I don't know what happened... I messed up one of the most simple recipes in the world. My Rice crispy treats turned out like a rice crispy rock. Of course I will place all the blame on the marshmallows; they were getting old but I insisted they will still be good for rice crispy treats. Oh well... it still tastes good and if you stick them in the microwave for a few seconds, they become gooey and mostly normal like again...