Snowballs are Calvin's winter weapon of choice. Wintertime generally leads to him spending a great deal of time stockpiling snowballs and throwing them at Susie and Hobbes. Likewise, Susie and Hobbes stockpile snowballs to throw at Calvin.
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 giant snowballs.
Normally when Calvin throws a snowball at Susie, she responces by beating him up and tossing him in the snow, even on one occasion 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 sees Susie walking by, he quickly retrives the snowball. 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 December, Calvin is deciding if he can throw the snowball, due to his belief in Santa Claus, but he mostly throws them anyways, usually at Susie.
One time he made snowballs that filled up a wheelbarrow, but when he tries to throw them at Susie, every snowball misses.
Calvin usually makes his snowballs with extra things thrown in, like slush, ice, dirt, sticks, etc. However, Susie and Hobbes just make snowballs by packing snow together.