Door in a Jar was one of three inventions by Dr. Bunsen Honeydew which he gave to the Muppets in their attempt to rescue Gonzo from the C.O.V.N.E.T. labs in the 1999 film Muppets from Space. As Bunsen explains, "All you do is open up the jar, fling the contents onto the wall, and poof! Instant door."
Trapped in a hallway with security guards searching for them in the C.O.V.N.E.T. complex, Kermit asks Fozzie to use the Door in a Jar. Fozzie reads the instructions, then opens the jar and throws the green slime onto a wall. The slime drips down the wall and creates a tiny doorway. The Muppets are disappointed, but it's just the right size for Rizzo and Pepe, the latter of whom slips through the door and finds an escape route.
The doorway created by Door in a Jar looks a lot like the familiar brownstone stoop of 123 Sesame Street.