- Add a heaping teaspoon of yeast to 1/4 cup of warm (~110◦ F) water in a pyrex measuring cup
- Sprinkle on some sugar
- Leave alone for a few minutes, until a foam forms and it smells all yeasty. It's a good idea to do this before you start making the rest of the cake; it should be ready by the time you get all the other things up and running.
- Use milk to fill the measuring cup the rest of the way (to whatever amount of water the box calls for) - be sure to measure from underneath the foam NOT on top of it
- Proceed as directed
*My mom has a master's degree in home ec. This is a trick she picked up back in the day - I imagine cake mixes were much more nasty back then.