system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are contacts so thin when eyeglasses are so thick?
gpt
The important thing is the size of the lens. A lens needs an even curve from one edge to the other. This curve continues from the middle all the way to the edge, becoming steeper as it goes, and the steeper the curve gets, the thicker the lens is.

But just as a larger lens becomes much thicker, then a smaller lens becomes much thinner. The active part of a contact lens is the size of the opening of the eye, the pupil, which seldom gets wider than 8mm. A pair of eyeglasses is normally at least 30mm across, sometimes as wide as 50mm.

To get an idea of this, take a look at the middle 8mm - less than ¾" - of a glasses lens. Imagine chopping just that bit out of the lens. Either that bit will already be thin, or it will have fat edges all around, and you could subtract all that thickness without changing how it works. Then notice how the curve of the glasses becomes greater from there to the edge of the glasses. This is where all the thickness comes from.