The Ice Temple is full of dangerous things for Fireboy and Watergirl to face. You must do a lot of things right to get these brave heroes safely to the exit.
Switch between Fireboy and Watergirl, but be careful. Since fire and water don't mix, don't let Fireboy go in the water lakes and don't let Watergirl go in the fire lakes. And don't let either of them go into the black lakes.
In the options menu on your phone, you can change how the controls work.