I ran across these foam blocks at Michael's crafts and the red flags started waving! They are light, slightly bigger than dice, and cheap cheap cheap cheap cheap. I decided to write the names of the music alphabet on them and use them to demonstrate scale building, intervals triads, and oh....lots of things. Sometimes when we are reviewing keys and key signatures, I get the old question- "Why isn't there a key of D# major?" These little blocks are a handy answer.
When placed on the keys of the piano (by the student, of course), in alpha order, following the whole, whole, half, whole, whole, whole half pattern of the major scale, the answer becomes instantly clear. We don't have an Fx or Cx in our major/minor system. It just doesn't allow for double sharps (or flats) in a major key. Besides....we already have the enharmonic of E-flat major, so why not just use that? Its a lot easier juggling an F and G, than an E# and Fx.
You can also have the student set the blocks of E-flat major right next to the yucky D# major so they can see for themselves. I've got lots of visual learners, and sometimes this is exactly what they need to "see" and understand it.