Insignificance

Roosa (up front) is lecturing to grandson Junnu about the
relative and also absolute insignificance of tomcats.

He took refuge on my shoulder soon after that.

It's mostly a harmless game but about once a month they
clash with some hissing and screaming.

It's likely because of Junnu, who likes to stalk on Roosa and take her by surprise - for which a noisy showdown ensues.