I'm having an issue with replying to comments so here s the short, for fun version of making Ivory 'snow' in the microwave:
Break the bar of soap in half or more pieces. Place on a papertowl in the microwave. Use HIGH power and set the timer to 2 minues. Watch the soap expand. Due to variances among microwaves exact time varies. Keep watching so you do not burn/overheat.
When microwave finishes let set for a minute to cool before touching.
I've printed directions to send with the soap including more scientific explanations for those who homeschool or otherwise would like to make this an educational.activity.