I heard that one of them you need to add sugar to feed the yeast...
Amanda is a developer at Food52.
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Basically, instant yeast (aka rapid rise) can be used without hydrating it first.
Feeding with sugar ("proofing") isn't necessary in either case, basically a holdover from days past, a test to "prove" the yeast was still active.
Here is a brief treatise on baking yeasts that may answer your question: http://www.kingarthurflour...