Each Flip Coin has a number of stars in different categories. If a question comes up you don't know the answer to, you can use a Flip Coin that matches the category of the question.
To choose a Flip Coin you need to press the small round button above the question. This will bring up a list of the Flip Coins which are available.
Swipe up the screen to flip the coin you chose. If you flip Heads, the correct answer to the question will be revealed. If it comes up Tails you might be able to try again.
The number of stars a Flip Coin has in a particular category indicates the number of times you are able to flip the coin to try and get Heads.
Once you've used a particular Flip Coin, you'll have to wait a while for it to "cool down" before you are able to use it again.