Heat capacity of water.
Water is able to absorb a lot of heat. In this experiment we held an empty inflated balloon over a flame and it popped immediately. We then held an inflated balloon, 1/4 filled with water in the flame. No matter how long we held the balloon in the flame it wouldn't pop.
We had the balloon in the flame so long it got soot marks on it.
The balloon is safe because water is able to absorb the heat from the flame so the balloon doesn't melt and tear. I have heard you can hold the balloon over the heat until the water starts to boil and it still won't break. We only held it for a few minutes.