What does Jem teach Dill to do in To Kill a Mockingbird?

Jem taught Dill to swim.

Dill is a summer neighbor.  He comes to Maycomb every summer to stay with his Aunt Rachel.  He is best friends with Jem and Scout, and they are inseparable all summer.  Even though Jem is older and Scout is a girl, it doesn’t really matter. 

There are times, however, when Jem and Dill go off by themselves and Scout is not included.  She does not really think of herself as...

Jem taught Dill to swim.

Dill is a summer neighbor.  He comes to Maycomb every summer to stay with his Aunt Rachel.  He is best friends with Jem and Scout, and they are inseparable all summer.  Even though Jem is older and Scout is a girl, it doesn’t really matter. 

There are times, however, when Jem and Dill go off by themselves and Scout is not included.  She does not really think of herself as a girl, so she is hurt when they leave her out.  One example is when Jem teaches Dill to swim.

Jem had discovered with angry amazement that nobody had ever bothered to teach Dill how to swim, a skill Jem considered necessary as walking. They had spent two afternoons at the creek, they said they were going in naked and I couldn’t come … (Ch. 24)

When Dill comes to Maycomb, the summer has officially started for Jem and Scout.  They spend the summer playacting and trying to get Boo Radley to come out until the summer of the trial.  Dill was supposed to stay home, but he came because his mother is remarried.  Dill ran away from home because his mother and stepfather ignored him.

He had taken thirteen dollars from his mother’s purse, caught the nine o’clock from Meridian and got off at Maycomb Junction. He had walked ten or eleven of the fourteen miles to Maycomb, off the highway in the scrub bushes lest the authorities be seeking him, and had ridden the remainder of the way clinging to the backboard of a cotton wagon. (Ch. 14)

This is an example of how badly Dill needs to be in Maycomb.  Maycomb and the Finches mean everything to him.  It is more than his second home.  The Finches are his family, because he has a lonely home life.  No one even ever taught him to swim.  Dill needs Scout and Jem as much as they need him.