If you have old yeast, you want to test it to make sure it is still viable before using it to make bread. To test the viability of yeast, stir together 1/2 a cup of warm water with a teaspoon of sugar. Sprinkle on a packet (or 2 1/4 teaspoons) of active dry yeast. Let sit for 5-10 minutes. If the mixture gets bubbly and puffs up, the yeast is still viable. If it doesn't change, then it is dead and you'll need to buy fresh yeast.