At regular intervals throughout the movie, the hosts leave the theater and return to the bridge of the satellite to perform sketches (commonly called "host segments") that often satirize the film being watched. The general format of an "MST3K" episode has remained the same throughout the series' run. Episodes are approximately 90 minutes in running time (excluding commercial breaks) and begin with a short introductory segment in which the human host and the 'bots interact with the Mads before being sent the movie. During Joel Hodgson and Jonah Ray's tenures as hosts (and for a brief period at the start of the Mike Nelson era), the hosts and the Mads engage in an "invention exchange" in which they each show off their latest inventions. Sirens and flashing lights ("Movie Sign") then signal the characters to enter the theater. In the theater, the human host and 'bots' Tom and Crow sit in a row of theater seats, shown in silhouette along the bottom of the screen, an approach Hodgson called "Shadowramma". The three then riff on the film (which is sometimes accompanied by one or more shorts) as it plays for both them and the audience. Occasionally the silhouette format is used as a source of humor or as a means of creating unobtrusive censor bars for scenes containing nudity. The show transitions into and out of the theater via a "door sequence", a series of six doors that open or close as the camera (ostensibly Cambot) passes through them. At regular intervals throughout the episode, the characters leave the theater and perform sketches usually inspired by the events of the film or short being shown, frequently making use of original songs and prop comedy. Some sketches bring in new or recurring characters or other devices; the host would consult an external camera "Rocket Number Nine" to show events happening outside the "Satellite", and the "Hexfield Viewscreen" would be used to communicate with other characters from the ship's bridge. At the end of each sketch, "Movie Sign" is triggered again and the characters must re-enter the theater. During Hodgson's period on the show, the final sketch aboard the "Satellite" often included reading of fan mail from the "MST3K Info Club". Fan mail readings decreased during Mike Nelson's tenure as host and were dropped entirely once the show moved onto the Sci-Fi Channel. The final sketch of an episode typically ends on the Mads, with the lead Mad asking their lackey to "push the button" to end the transmission and transitioning to the credit sequence.