They chew holes in rocks.

Limpets, in the wild, often grind out circular holes in the rocks, "nests" to sleep in while the tide is out. I've read that they do this by scraping with their shells, but how does a soft carbonate shell scrape a hole in granite?

They use their teeth. Microscopic teeth set in soft limpet flesh scrape away even the hardest rock.

My teeth hurt, just thinking about this.

A new study from the University of Portsmouth, published just last month, has calculated the strength of these limpet teeth, and found that they are harder than steel, stronger than spider silk, resist breakage better than diamond.

(I found the study because a report on the study used my photo as illustration.)

Limpet chewing algae on glass.

The tiny teeth on a conveyor belt structure in the mouth, owe their strength to fibers of goethite, an iron-bearing mineral found in soil.*

High-magnification electron microscopy images of the tooth cusp. From RSIF.

From the Conclusion of the study:

We show that the tensile strength of limpet teeth can reach values higher than spider silk, considered currently to be the strongest biological material, and only comparable to the strongest commercial carbon fibres. ... As the limpet tooth is effective at resisting failure owing to abrasion, as demonstrating during rasping of the tooth over rock surfaces, corresponding structural design features are expected to be significant for novel biomaterials with extreme strength and hardness, such as next-generation dental restorations.

Zooming in on a limpet's mouth, showing the radula, with its scraping teeth.

I looked up tensile strength values, because the report said the limpet's teeth were 5 times stronger than spider silk, and I didn't find those numbers in the study. I found them, eventually, on Wikipedia. Here's a selection from their table:

Original, complete but long, table here.

* And now I'm wondering how they get the goethite in their teeth. Do they find it in the soil and eat it, like some nudibranchs borrow venom from anemones? Are they born with it? I wonder if there's a study on that?

(Study source, including photo:  Extreme strength observed in limpet teeth
Asa H. Barber , Dun Lu , Nicola M. Pugno
DOI: 10.1098/rsif.2014.1326 Published 18 February 2015

Under a Creative Commons license)