A student put a half teaspoon of yeast in each of three different balloons. Then he added a quarter cup of water to each balloon. Each balloon received water of a different temperature: hot, warm, and cold. After adding the water, the student tied off the end of each balloon to seal it shut. The yeast began to produce bubbles of gas inside the balloons. After 20 minutes, the student measured the circumference of each balloon to determine how much it had grown in size.

What is the responding variable in this experiment?