“thorns underneath a pot.” In this verse, the rhyme grabs the attention and causes one to pause and think about the saying. The word used here for “thorn” and “pot” are homonyms, they are spelled the same but have different meanings. The word sir (#05518 סִיר or )fem.סִירָה ( sirah), means “pot,” a household pot, and also means “thorn,” “briar,” or “hook.” The Hebrew is hasirrim tachat hasir (הַסִּירִים֙ תַּ֣חַת הַסִּ֔יר), “thorns [plural] underneath a pot.” There is really no way to reproduce the rhyme and wordplay of the Hebrew text into English, although some scholars have tried. One such attempt is, “the sound of nettles under the kettle” but that falls far short of the Hebrew.
Since everyone cooked over a fire, and since it was common to burn thorns, everyone was familiar with the characteristics of burning thorns. There are a couple very distinct things about thorns when they burn. Thorns burn loudly; they make a loud crackling sound when they burn. Similarly, fools are generally loud and obnoxious. Also, thorns burn up quickly and then are gone. Similarly, fools may laugh now, but they soon pass away and then are gone forever. They foolishly reject God and so do not have everlasting life. They are fools, so why listen to them? It is much better to listen to the rebuke of a wise person (Eccles. 7:5).
“pointless.” See commentary on Ecclesiastes 1:2.