General thread question!

[spanielpup]

How does fly tying thread differ from ordinary sewing threads?
I was in John Lewis recently and noticed they have hundreds of threads made of lots of different materials/ thicknesses and in just about every colour under the sun. My wife also tells me that there are even finer threads used in machine sewing. I would have thought they can be used for tying flies but just wondered if you would need to do anything to them?
My wife was once quite into all things sewing and embroidering and there seems to be a fair bit of stuff left over in her sewing box which I wondered about using.

[BRUCE1]

most tying threads are polyester whereby they wont rot ,sewing threads are generally of cotton which would rot .

[flyinghigh]

Also sewing thread will turn black after being soaked in water.

[shuck raider]

We did this a while ago. Some of the replies here might answer your query

[spanielpup]

Thank you all. Very informative. I asked the question mainly as yhe Unithread 8/0 Olive keeps on snapping which makes it quite difficult if you are trying to pull a bit tighter. I found a similar colour in my wifes box and simply wondered if it was a bit stronger. No Probs - yhanks for the Help!