What causes fingernails (not nail polish) to peel?

I was curious as to what this could be..

this could be it:

Brittle Nails are characterized by a vertical splitting or separation of the nail plate layers at the distal (free) edge of the nail plate. In most cases, nail splitting and vertical ridges are characteristic of the natural aging process. This nail problem is also the result of overexposure to water and chemical solvents such as household cleaning solutions. As we age, the nail bed's natural flow of oils and moisture is greatly reduced. This oil and moisture is the cement that holds the nail plate layers together and gives the plate its inherent flexibility. At the first signs of splitting or peeling, re-hydrate the nail plate layers with a good quality cuticle and nail oil that contains Jojoba and Vitamin E as two of the botanical oils. Jojoba oil has a very tiny molecule which can penetrate the nail plate surface, open up the layers and draw the Vitamin E in after it. The molecular structure of Vitamin E is too large to penetrate the nail plate layers or the surface layer of the skin without the benefits of Jojoba oil. Oil the nail plate and surrounding cuticle at least twice daily; more if you have your hands in water a lot. Wear gloves whenever working with household cleaning solutions, and remember: water is considered the 'universal solvent', and is indeed a 'chemical'.

if you have dry skin under the nail it can also cause the fingernail to peel.

lack of calcium

Fingernails peel not only as a result of poor nutrition and/or skin care, they also peel as a symptom of the medical condition, Hyperthyroidism. A simple at home test that doesn't necessarily diagnose hyperthyroidism but added to the symptom of nail peeling or flaking might warrant further examination. For this test: Hyperextend your arm in front of you, palm side down. Place an 8.5x11" piece of paper on top of the back of your hand. Does the paper shake? If so, you may need a thyroid uptake. If not, take some vitamins, eat some jello and condition your skin & nail bed.

Look after your nails and replace the lipids you strip from them daily by washing your hands or just living in a hot / cold dry environment.

Watch this video of a fingernail being hit by a HAMMER.
http://www.youtube.com/watch?v=THEiIVd19oQ