By typing in Rickroll a character will appear that looks like Rick Astley. He will pose, then explode after a few seconds.
A glitch can be performed with Rickroll that will allow you to put an infinite amount of objects in a level. This glitch will work in both Challenge Mode and Free Play Mode, but not the level editor. To perform the glitch, write Rickroll and put him on the ground and then immediately pick him up again and wait a few seconds. When he explodes, his sprite will remain on the stage but the bar on the top screen that represents how many objects you can write will go down. Then drag Rickroll to the delete bin on the top left of the bottom screen and delete him. The game will register Rickroll's deletion twice and the bar will be smaller than when you first wrote him. You can repeat the glitch until the bar is empty, even if there are objects in the level. You can also do the same thing with DO NOT WANT and SPAGHETTI CAT.