In comedy, a Sight Gag is anything which conveys its humour visually, often without words being used at all.
Sight gags are often used in surreal comedy, with many Monty Python's Flying Circus sketches making use of them, such as the "Mrs Gorilla" sketch in which a series of middle-aged women have been shopping and bought piston engines. Likewise, many elements of the "Hell's Grannies" sketch, featuring Keep Left signs attacking passersby, are sight gags.
As Rowan Atkinson explains in his lecture Funny Business, an object or a person can become funny in three different ways. They are:
By being in an unusual place
By behaving in an unusual way
By being the wrong size
Most sight gags fit into one or more of these categories.
This type of sightgag is set up so that a character in the film misunderstands something that is happening in the scene, however the audience can see both sides of the situation and is aware of the truth.
The definition of a switch movement sightgag is when "one movement is transformed into another in one seamless line of movement."(Page 35) With this technique, it is possible for a character to completely change the way one action looks by smoothly shifting into another action.
The author points out that "the sightgag flies in the face of the prejudice that movies can only brutishly recapitulate from a single point of view what stands before the camera." From the conflicts of interpretation to the literal and metaphoric points of view, it is these things that confuse but amuse us.