I have some yeast in the fridge with an exp. date of Apr 12. I did not know if it would work so I put it in some warm sugar water and waited. IT GREW! After a big blob formed, I went ahead and added it to the fixin's for raised waffles/pancakes and it's popped the top of the Tupperware mixing bowl twice already and it has reached the top. It is actively bubbling like something at a slow boil! It is rising in my over-the-range MW oven. With the lights over the cooktop on, the inside of the oven is the perfect temperature for making yeast work.