How a Mosquito Operates is a 1912 silent animated short film by the American cartoonist Winsor McCay. The six-minute short depicts a giant mosquito tormenting a sleeping man. The film is one of the earliest works of animation, and its technical quality is considered far ahead of its time. It is also known under the titles The Story of a Mosquito and Winsor McCay and his Jersey Skeeters.

McCay had a reputation for his proficient drawing skills, best remembered in the elaborate cartooning of the children's comic strip Little Nemo in Slumberland he began in 1905. He delved into the emerging art of animation with the film Little Nemo (1911), and followed its success by adapting an episode of his comic strip Dream of the Rarebit Fiend into How a Mosquito Operates. McCay gave the film a more coherent story and more developed characterization than in the Nemo film, with naturalistic timing, motion, and weight in the animation.

How a Mosquito Operates had an enthusiastic reception when McCay first showed it as part of his vaudeville act. He further developed the character animation he introduced in Mosquito with his best-known animated work, Gertie the Dinosaur (1914).

A man looks around apprehensively before entering his room. A giant mosquito with a top hat and briefcase flies in after him through a transom window. It repeatedly feeds on the sleeping man, who tries in vain to shoo it away. The mosquito eventually drinks itself so full that it explodes.

How a Mosquito Operates is one of the earliest examples of line-drawn animation. McCay used minimal backgrounds and capitalized on strengths of the film medium, then in its infancy, by focusing on the physical, visual action of the characters. No intertitles interrupt the silent visuals.

Rather than merely expanding like a balloon, as the mosquito drinks its abdomen fills consistent with its bodily structure in a naturalistic way. The heavier it becomes, the more difficulty it has keeping its balance. In its excitement as it feeds, it does push-ups on the man's nose and flips its hat in the air.

The mosquito has a personality: egotistical, persistent, and calculating (as when it whets its proboscis on a stone wheel). It makes eye contact with the viewers and waves at them. McCay balances horror with humor, as when the mosquito finds itself so engorged with blood that it must lie down.

Winsor McCay (c. 1869–1934) developed prodigiously accurate and detailed drawing skills early in life. As a young man, he earned a living drawing portraits and posters in dime museums, and attracted large crowds with his ability to draw quickly in public. McCay began working as a full-time newspaper illustrator in 1898, and started drawing comic strips in 1903. His greatest comic-strip success was the children's fantasy Little Nemo in Slumberland, which he launched in 1905. McCay began performing on the vaudeville circuit the following year, doing chalk talks—performances in which he drew in front of a live audience.