Final answer:
Ender chuckles to himself while in zero gravity and on his way to Battle School because he sees the humor in the absurdity of the situation.
Step-by-step explanation:
Ender chuckles to himself while in zero gravity and on his way to Battle School because he sees the humor in the absurdity of the situation. This is represented by the example of astronauts in space who appear weightless due to falling freely around the Earth, similar to Ender's situation. The feeling of weightlessness and the unusual circumstances of being in zero gravity can be amusing to Ender, leading him to chuckle.