The Prince and Tom are mistaken for each other at the beginning of the play because

Answer: They tried on each other's clothes.

Explanation:

In the book, The Prince and the Pauper by Mark Twain, Tom the pauper and Prince Edward Tudor exchanged clothes and were startled by what they saw. They were very much alike but for the clothes they wore and when they changed clothes it was hard to tell them apart. Even the guard at the gate could not tell them apart and even slapped Edward thinking it was Tom.