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, "Haw haw! 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.
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.
When Nelson appears as a young Beethoven, his laugh has four "Ha"s and he sings it so that it sounds like the opening notes of Beethoven's Fifth Symphony: "Haw-haw-haw-haaaaaaaw, haw-haw-haw-haaaaaaaw!"
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: "Haw-Haw?" 
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 "Haw Haw!.
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
- 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. 
- Nelson laughs at a tall man driving a Volkswagen, 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.
- ↑ I, D'oh-Bot
- ↑ 2.0 2.1 The Simpsons Movie
- ↑ 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