Here we go:

Larson took the unusual step of issuing a press release, explaining the joke:

The cartoon was intended to be an exercise in silliness. While I have never met a cow who could make tools, I felt sure that if I did, they (the tools) would lack something in sophistication and resemble the sorry specimens shown in this cartoon. I regret that my fondness for cows, combined with an overactive imagination, may have carried me beyond what is comprehensible to the average Far Side reader.[1]

Years later, in The Prehistory of the Far Side, Larson further explained that he was inspired by the idea that tool use was the characteristic that separated mankind from the rest of the animal kingdom

I mean, ok.