Snowballs are Calvin's weapon of choice during wintertime. Wintertime generally leads to him spending a great deal of time stockpiling snowballs and throwing them at Susie and Hobbes, once at his Dad. Likewise, Susie and Hobbes stockpile snowballs to throw at Calvin.
Calvin once claimed to Hobbes, "Everyone's a slave to efficiency, most people lump snow together and call it a snowball." He goes though a complicated process of making his snowball, including signing them, only to get pounded by Susie using four quickly made snowballs.
Calvin usually complains about when he misses. He blames his mittens, or a cross wind, and/or the snowball itself. Hobbes and Susie normally use this time to retaliate with another snowball. When it doesn't miss (usually followed by several boasts), they may do other things such as dunking him into a giant snowball.
Normally when Calvin throws a snowball at Susie, she responds by beating him up and tossing him in the snow, even when Calvin missed.
In one strip, Calvin decides to freeze a snowball in the kitchen freezer, to use against Susie in the summer. When he finds that Susie is playing on the sidewalk, he quickly retrieves the snowball from the freezer. He sneaks up behind her, throws it, and misses. As he is berating himself, Susie quickly gathers up the snow and hits him in the face. In another, he stored snow in his thermos, brought it to school, and emptied it to make a snowball. In yet another, he placed a snowball in his desk, but when he opened it, he discovered it melted.
In December, Calvin ponders if he should throw the snowball, due to his belief in Santa Claus, but he mostly throws them anyways.
One time he made snowballs that filled up a wheelbarrow, but when he tries to throw them at Susie, every snowball missed.
Another time he made two snowballs and took them in the house to throw at his parents saying, "Incredibly, people never expect to be hit by a snowball in the house." Causing Hobbes to say, "I'll see you in your room, momentarily."
Calvin usually makes his snowballs with this recipe, "1/2 snow, 1/4 ice, 1/4 slush, and a dash of assorted debris." However, Susie and Hobbes just make snowballs by packing snow together.