Peanuts by Charles Schulz for December 20, 1965
Transcript:
Charlie Brown stands behind a snow fort, next to a small pile of snowballs. Charlie Brown says, "Ready?"<BR><BR> Charlie Brown shouts "Ready?" again.<BR><BR> Charlie Brown shouts, "Ready?"<BR><BR> Linus stands in front of a small mountain of snowballs and replies, "Ready."<BR><BR>
Pretty sure Calvin and Hobbes had an almost identical strip, decades later.