S3·E9 The Search for String

Nick and Sally need a strong piece of string to pull their train, but it breaks, and they don't have anything stronger in their craft box. The Cat takes them away, but the Thingmajigger malfunctions and they have to make an unscheduled landing. It turns out they've shrunken and are inside the craft box, where they discover that they can create a stronger string by braiding several strands together.