Water balloons are Calvin's favorite summertime weapon, much like snowballs are during winter. His most common targets are Susie and Hobbes. During the summer, he often has wars with Susie or Hobbes, or else makes plans with Hobbes to attack Susie. (These plans sometimes fail.) Susie seems to have learned to expect this however, as she is seen in one strip with a raincoat and umbrella on, as Calvin prepares to throw a water balloon at her; to which he shouts, "YOU THINK YOU'RE SO SMART!"
Sometimes, Calvin has plans to attack other people; for example, in one strip he's hanging a water balloon from a tree. He then acknowledges that the string is too small and asks his dad how tall he is.