Ha Ha! is Nelson's catchphrase and distinctive laugh. Nelson says the laugh with the emphasis on the first "Ha," so it sounds like "HA-ha!"
Nelson usually uses the laugh when something bad happens to another character. Sometimes, after he uses the laugh, he says why he is laughing at the person. For example, he says, "Ha ha! Your dad's not handy!" to Bart after his ten-speed bike (which Homer had assembled) fell apart while Bart was riding it.
Sometimes, Nelson laughs for so long at a time that his voice gets really tired and croaky, which was shown in the Simpsons Movie, when he laughs at Bart..
When Nelson laughs at Martin shortly after punching him in the gut when the latter's birthday party was forced to end early due to the guests falling ill from food poisoning from oysters served by Mrs. Prince, Nelson proceeds to do his trademark laugh, only for his second "ha" to be drawn out into more of a pained groan as he himself succumbs to food poisoning.
In “Bart the General”, Nelson's first appearance, he laughs with three "Ha"s, particularly when Bart imagines Nelson as a giant who eventually eats him up, and in “The War of the Simpsons” he laughs like this to Grampa Simpson.
Sometimes, Nelson will start the laugh, and then partway through realize that the joke is actually on him, and the laugh comes out sounding like a question: "Ha-Ha?" 
The Irish Nelson says "Har! Har!"
When Bart jumps on the see-saw and the rubbish bag on the other side lands on him, Nelson says "See-Saw" in the same sort of way as "Ha Ha!.
Nelson trips up Facebook founder Mark Zuckerberg in a roller-skating rink. Zuckerberg falls and hurts himself and updates his status on Facebook to "in severe pain". Nelson replies, posting "Haw Haw!"
Uses by Other Characters of Simpsons
- Mrs. Muntz has been known to use the laugh.
- Cypress Hill member B-Real says "Ha-ha!" when he overhears Marge referring to Bart wetting his pants at the Christmas play. 
- Lisa also says it in the episode The Haw-Hawed Couple after discovering that Bart and Nelson are best friends.
- Nelson laughs at a tall man driving a Volkswagen Beetle, who turns the tables on him. The man makes Nelson walk down the street with his pants pulled down around his ankles, while everyone in Springfield laughs at him.
- In Who Shot Mr. Burns? Part One, Nelson laughed at Santa's Little Helper's misfortune at the Vet's, his parrot also used the laugh.
- ↑ I, (Annoyed Grunt)-Bot
- ↑ 2.0 2.1 The Simpsons Movie
- ↑ Grade School Confidential
- ↑ El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer)
- ↑ Margical History Tour
- ↑ Boy-Scoutz 'n the Hood
- ↑ Rome-Old and Julie-Eh
- ↑ Boy Meets Curl
- ↑ In the Name of the Grandfather
- ↑ The Debarted
- ↑ "Loan-a Lisa"
- ↑ Homerpalooza
- ↑ 22 Short Films About Springfield