I've failed to grasp the meaning of this sentence. Can you help me?

"Theophilus sifted three thousand thistles by the thick of his
thumb."

Question

I've failed to grasp the meaning of this sentence. Can you help me?

"Theophilus sifted three thousand thistles by the thick of his
thumb."

Answer

This is a tongue twister, I presume.

It is grammatically correct, but it doesn't mean anything. It is just a sentence that is difficult to pronounce and is therefore something good to practice with.

Answer

Феофил просеял (буквально: руками протер через сито) три тысячи головок чертополоха подушечками большого пальца. Вообще говоря, это скороговорка и большого смысла не должна иметь, не более чем "шла Саша по шоссе и жевала сушку"