Making and using the tube makes a creative activity for the weekend - in an hour's work, you could make enough knitting pipes to keep your kids, and all their friends, constructively occupied for ages.
I used power tools and a blowtorch, but you could also use hand tools and sandpaper.
As a bonus, I'll also explain how to use the pipe. Even better, there is now a 3D printed upgrade to the idea (see step 7)!
Step 1: Materials
You could use any tube you like, as long as you have the tools to deal with it.