What did Stargirl put inside her happy wagon?

In chapter 22, Leo discovers that Stargirl puts pebbles in her happy wagon. 
The happy wagon sits on Stargirl's book shelf. The wagon is about the size of Leo's fist, and it is made of wood. When Leo questions Stargirl about it, she admits that the happy wagon serves as a barometer of her mood.
When she is happy, she puts a pebble in the wagon, and when she is sad, she takes a pebble out. Stargirl tells Leo that she has twenty pebbles in all. At the moment, she has seventeen pebbles in the wagon, which means that she is pretty happy with life.
When Leo asks about the largest number of pebbles she has had in the wagon, Stargirl informs him that it is seventeen. Leo is touched by her confession. He then questions how close to empty the wagon gets. 
Upon hearing this, Stargirl admits that she went down to three pebbles once. This admission surprises Leo, as Stargirl is known as the proverbial happy girl at school. The chapter ends with Stargirl telling Leo that she now has eighteen pebbles in her happy wagon, as she leans in to share a kiss with him.