Sometimes the recipe says bring to a simmer, while the pot is over very low heat. It seems like it would take a very long time to come to a simmer over this heat, if ever. Does liquid always come to a simmer eventually, no matter what amount of heat it is over? Or can you crank it up, bring it to a simmer, and then return it to its original level of heat? If it isn't over heat and the recipe says "bring to a simmer" should you use high heat, or medium heat?