String is supplied in some K'NEX sets, and can also be bought separately in our K'NEX shop.
If you are using string with younger children, they may find it difficult tying knots in the string to attach it to their model. One solution is to tie each end of the string through the hole in a grey connector for them, as shown in the photo, so that all they have to do is fasten the grey connectors on in the right place.
String is very useful to transfer a pulling force from one part of the model to another, or for transferring a turning force into a pulling force - see photo.
You will probably find that after using string a number of times, it becomes tangled or frayed, and will need replacing.