In Chapter 2, Todd Amos is beating up Bud when Mrs. Amos walks into the bedroom. Todd lies to his mother and fakes an asthma attack. Todd points to Bud's hand mark on his face, and Todd's parents make Bud sleep in the ominous looking shed. After Bud escapes a swarm of attacking hornets in the shed, he sneaks back into the Amos' house to get revenge on Todd. Bud quietly fills a glass with warm water and sneaks into Todd's room while he is asleep. Bud puts Todd's fingers in the glass and hopes that it will make Todd pee his bed. This doesn't work, and Bud decides to pour water over Todd's hand to get him to pee the bed. After another failed attempt to make Todd pee, Bud pours the warm water on Todd's pajama pants. Bud's final attempt is a success, and Todd Amos pees his bed. Todd had accused Bud of looking like a bed-wetter in front of Mrs. Amos earlier, which embarrassed Bud. Bud understands how embarrassing it is to be labeled a bed-wetter, and he decides to seek revenge on Todd for sticking a pencil up his nose, beating him up, calling him Buddy, and lying to Mrs. Amos who made Bud sleep in the shed.

What did Bud do to Todd in Bud, Not Buddy? Why did he do this?

Bud slapped Todd Amos because Bud was upset about Todd ramming a pencil up his nose. This particular event happens in chapter 2. Bud has just been adopted from the orphanage, and he quickly finds out that his new big brother is a bully. Bud tells readers that he woke up to find that Todd Amos was shoving a pencil up Bud's nose while he slept. Todd is amazed at how far up he could get the pencil. Todd laughs about the incident and then calls Bud "Buddy."

Bud was mad about the pencil, but being called "Buddy" pushes him over the edge. Bud takes action and takes a swing at Todd's head. Unfortunately, Bud doesn't keep his fist closed, so his punch ends up being a slap. Todd's mother eventually enters into the fray and punishes Bud by locking him in the shed.

It is possible that this question is asking about what Bud does to Todd in chapter 4. Bud breaks himself out of the shed, and he then sneaks back into the Amos household in order to retrieve his suitcase. While in the house, Bud fills a container with warm water and sneaks into Todd's room. Bud's goal is to get Todd to urinate in bed, since Todd told an earlier lie that Bud was a bed wetter. Bud tries multiple times to get Todd to wet the bed, and Bud is finally successful after pouring the warm water on top of Todd's pajama pants.

He smiled and the warm water from the jelly jar opened that little valve up and ... woop, zoop, sloop ... he soaked his sheets!