a) they get dirty
b) I have to wear socks over them, which is a fouter, and rather warm.
Enter Dylon! And, since it's a waste of a packet to only dye a little, I did a couple of other things while I was at it.
Two very faded pillowcases got tie-dyed, and one T-shirt which had mysterious mildew marks, but is fine for working. I fact, I really like it now.
A fun evening, and a new lease of life for some things, all for £3.20.