Note to the author -- I love the interactive part. One suggestion to really drive the point home: Give the user an example of two strings that have a hash coll

Note to the author -- I love the interactive part. One suggestion to really drive the point home: Give the user an example of two strings that have a hash collision, and tell them to enter one into the box, and then test for the other in the second box. It will demonstrate why the answer is always "maybe it's in the set!" instead of "yes".