Helen Deborah Vecht wrote:

"JNugent" typed

Why?
Do rucksacks also have hardened steel sharp bits sticking out and/or
covered in black grease as well?

Rucksacks have aluminium frames that are almost as hard. They can seem
to move in the opposite direction to their owner.

Not too welcome on the Northern Line during the rush-hour, I should think.

There ain't much grease on a unicycle, cos most don't have chains...

But some do.