Question: what is a cataract and how does it form?

Steven Gardner answered on 19 Jun 2013:

I answered this question yesterday so I’ll copy my answer here.

The lens in your eye is mostly made of small chemicals called crystallins. These are crystal-like proteins, as you age these proteins condense and harden. This harder lens begins to scatter incoming light, and so it appears cloudy.

Cataract surgery these days is basically a transplant, the surgeon cuts into your eye and removes the old hard lens, and replaces it with a fresh clear one. Before they could transplant though they used to poke out the lens with a long needle!