Always getting the message that "YOu have lingots, next time you should consider getting a heart re-fill", but I am not given the opportunity to do that. If I click on the lingots the program takes me away from my session, and when I come back it has restarted me to the beginning just as if I had had four errors.
You have to buy a heart refill before you start a lesson. Once you have 'purchased' a heart refill, when you start a lesson you will have an option to refill one heart at anytime during the lesson. This can be a bit tricky because you have to use it before you run into hearts. If you are down to zero hearts, and think you may miss the next question, you can use a heart refill to refill one heart.
In doing this, sometimes you will use a heart refill when you don't need to and sometimes you will run out of hearts if you are feeling confident and don't activate the refill and run out of hearts (the heart refill stays in your inventory until you use it)