For as long as humans have had hair, we’ve had company—tiny, bloodsucking freeloaders called head lice. These sesame seed-sized pests have been a part of human history for at least 10,000 years, and despite our best efforts, they just won’t go away.

Lice don’t jump, fly, or hitchhike on hats as much as people think. They spread mostly through head-to-head contact, crawling from one scalp to another in search of their next meal. Once settled, they pierce the skin, inject anti-clotting saliva (hello, itchiness!), and start laying eggs, gluing them onto hair shafts with a waterproof, chemical-resistant sheath.

That’s why lice shampoos alone don’t always work—while they kill live lice, they leave the eggs intact. Scientists are looking for new ways to break down these stubborn nit sheaths, but for now, the best tool remains one as old as civilization itself: the fine-toothed nit comb.

Watch below to learn more about these stubborn scalp squatters!