1. It should be super strong, but not so strong that it can't break. Better to break your line than your fishing rod. It has to stretch somewhat, but not too much.

2. It should glide through your guides during casting and rewinding with minimal friction, cast far and accurately, then come back in with little effort. But it also has to allow the drag mechanism to work effectively, to put resistance on a fighting fish. It has to hold a knot well, too.

3. It should be light enough to cast far, but have no 'memory'. Memory is the feature of line that, for example, causes line to remain curled from being on the spool, kinked after being knotted.

4. It should be immune to cold, heat, sunlight, saltwater and slime.

And, oh by the way, all those (and many more) criteria come before even discussing that it has to be easy to make in large quantities and sell for only a few dollars. If a spool of fishing line could sell for $1,000 engineers would have a lot easier time designing it!

When working on the first goal, line designers have to carefully balance just when the line should snap. You don't want to let that good one get away, but you don't want to break a fishing rod when that big guy leaps up and runs away. As part of that effort, fishing line comes in different 'test weights'. That is, it may be 4 lb, 10 lb, 12 lb, 20 lb and so on.

To satisfy the second goal, fishing line makers have to be sure that the line is abrasion resistant. It has to stand up well to scraping over rocks, the edge of the boat, and so forth.

To create a good knot, a fishing line has to have some inherent friction, though. Otherwise, the knot would slip right out the first time there was the slightest tug on it. Line also has to be somewhat flexible, lest the angler be forced to spend ten minutes trying to tie something that feels like wire. Imagine what a surgeon would have to go through tying stitches with bad surgical line, for example.

Creating line that has little memory is a whole combination of science and art all its own. Anyone has noticed, for example, that a string wound around a stick has no memory. Unwind it and it hangs limply. Plastic or nylon, on the other hand, may well be curled when it comes off the stick, especially if it was curled on in the sun and unwound after it cooled in the shade. Once kinked it stays kinked.

One aspect of creating line with no memory is to try different materials, manufactured in various ways. Materials scientists spend years experimenting to get just the right balance.

At the same time, those same scientists are looking for materials that don't wear out from exposure to UV light, extreme temperature changes, being dipped in water that contains salt and other chemicals. They may devise a monofilament or a braid to add geometry to help that material do its job.

So, next time you're asked to design fishing line, just say "No, thanks" and be grateful there are so many good ones on the market to choose from already.