A transmogrification in process.
The transmogrifier was 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 original transmogrifier
The original transmogrifier.
The original transmogrifier first appeared on March 23, 1987. It was 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. Extra space was left on the side fo 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).
The transmogrifier gun
Main article: Transmogrifier Gun
The redesign of the transmogrifier used a water gun instead of a box and first appeared on February 8, 1988. Since there was no room for a dial, Calvin designed the transmogrifier gun to work on telepathy, so that the user could simply point the gun and fire. The target would then be turned into whatever he was thinking.
The transmogrifier gun.
The initial trial of the transmogrifier gun was less successful than that of its cardboard predecessor. Hobbes managed to use it to turn Calvin into a pterodactyl, but the result was only about a foot tall. In an ensuing fight, the pair wore out the new transmogrifier gun transforming each other randomly into different animals. Calvin managed to turn Hobbes back into a tiger, but the transmogrifier gun then broke, leaving Calvin as an owl. Fortunately, the effects of the transmogrifier gun are only temporary, and he turned back into a boy overnight.
The gun must have cooled off - or Calvin fixed it - because he used it in a later strip in which he floated away on an out-of-control toy balloon which eventually popped. During his long fall back to earth, Calvin discovered the transmogrifier gun in his pocket and proclaimed, "I'm safe!" The transmogrifier gun took him literally, turning him into a large floor safe. Fearing he would be late for dinner, Calvin finally transmogrifies himself into a light particle to return home at the speed of light.