A transmogrification in process.
The transmogrifier is an invention of Calvin's that would turn any one thing into another. Like most of his inventions, it was made originally from a cardboard box, though a later model was made using a water gun. Calvin used the transmogrifiers many times, turning himself and Hobbes into quite a wide array of creatures. The transmogrifier first appeared on March 23, 1987.
Appearance and effects
The transmogrifier is an upside-down cardboard box with a dial on the side that could be set to whatever the user wished to be turned into. The problem is that only Calvin and Hobbes can see whatever they turned into, so all the other people still see him as a kid. This was eventually fixed, however, because when Calvin turned his duplicates into worms, his mother saw them as worms as well. Extra space was left on the side of the box so that if the user wanted to turn into something not listed, it could be written in. The test run of the transmogrifier was largely a success, with Calvin being turned into a tiger for a day (or about a week's worth of strips).