Oh no," said Milo seriously. "In my family we all start on the ground and grow up, and we never know how far until we actually get there."

"What a silly system." The boy laughed. "Then your head keeps changing its height and you always see things in a different way? Why, when you're fifteen things won't look at all the way they did when you were ten, and at twenty everything will change again."

In addition to Milo's height, which will most likely change his perspective as he gets older?

his electric car
his experiences
his mystery package
his phantom tollbooth