Obtaining the Ice Flower will transform you into Ice Mario. This gives you the ability to throw iceballs that freeze enemies briefly. Frozen enemies can then be killed by performing a ground-pound or by throwing them.
You can also jump on them for extra height, which is valuable with flying enemies as they will be frozen in place. The effectiveness of this power-up varies depending on the enemy. Some will die after breaking out of the ice while others won't be effected at all.
