What is the difference between instant and active dry yeast?

The answer

The main difference is particle size: instant yeast has finer, more porous particles that absorb water fast, while active dry yeast has coarser granules with a protective outer layer. Instant yeast is usually mixed straight into the flour, and active dry yeast is traditionally dissolved in lukewarm water first. Both are the same species and work the same way. King Arthur and Red Star say they can be swapped one for one, and active dry may take slightly longer to get going.