Tellagami is a fun app to play with. It lets you create characters (gamis) and place them on a background, which can be one of the built-in backgrounds or something from your camera roll. You can then make a 30 second recording like the one above. You can talk to your students from space, inside a cell or even chat through a formula!
The app is free and there are in-app purchases to be able to customise your characters further, add backgrounds, or extend your recording to 90 seconds (but you can always join shorter movies together using iMovie). Basically there is a lot you can do with the free app! The full range of features is shown here.
See what teachers think of it here and here,
Tellagami is only available on mobiles (iOS and Android).