It was alive and moving, but it wasn't a snake, as Scout found out one night in Chapter 14 of Harper Lee's novel To Kill a Mockingbird. One night as Scout was going to bed, she stepped on something "warm, resilient and rather smooth."
...It was not quite like hard rubber, and I had the sensation that it was alive. I also heard it move.
Thinking it was a reptile, she called out to Jem, who was in the adjoining room. He went to the kitchen and returned with a broom. Ever the protector, Jem ordered Scout to get on the bed. Remembering Miss Rachel's encounter with a rattler in her own home, Jem carefully took a swipe and then another.
"Do snakes grunt?"
"It ain't a snake," Jem said. "It's somebody."
Indeed, it was only Dill, who had "escaped" from his folks in Meridian and taken a train to Maycomb Junction. Dusty and hungry, he had walked the remainder of the way to Maycomb,
Scout gets into a fight with Jem and is sent to bed early. She thinks there is a snake under her bed, so she rushes to get Jem who brings in a broom and starts poking under Scout's bed to find Dill who has run away from his parents.
This is a pivotal point of the novel for Jem, who gradually learns and matures as he witnesses the events going on around him. Jem breaks the "childhood code" and decides to tell Atticus about Dill, against Scout and Dill's wishes, because he knows that Dill's family will be worried and need to be notified.
Dill runs away from his mother and "new" father because they give him everything he could possibly want materially, but they deny him the one thing he truly wants from them: time and attention.