Ritesh Patel, of Riccarton High School, asks :-
Why do socks become easier to stretch when wet?
Stewart Collie, a textile technologist with the then Wool Research Organisation of New Zealand at Lincoln, responded.
Knitted fabrics (including socks) are made up of many interlaced loops of yarn. When they are stretched, the loops of yarn slide against adjacent loops of yarn and also change shape by bending and unbending. Water in a wet sock probably acts as a lubricant between the yarn loops (like the oil in the engine of a car), allowing them to slide more easily.
There is another factor at work for socks made from natural fibres (such as wool and cotton) because the fibres themselves become easier to bend and stretch when wet. Water enters the molecular structure of these fibres and interacts with some of the chemical bonds that hold the fibre together. Water doesn't interact with the molecules in man-made fibres (such as polyester or acrylic) so they are no easier to bend or stretch when wet.
Therefore, in wet wool or cotton socks there is a yarn lubricant effect and a fibre softening effect, while in man made fibre socks there is just the lubricant effect. Incidentally, natural fibres (and fabrics made from them) can be temporarily "set" into a new shape by deforming them when wet, then keeping deformed until dry. This is why cotton shirts are often wrinkled after washing.