don't stuff a rag underneath the saddle to wipe your hands with if you get dirty doing a repair, despite the "old timer" (my Dad) going on and on and insisting I did this, then sneakily stuffing a rag under the seat when I declined his advice.
When the rag falls out, catching in the chain, pulling the wheel out of alignment so it rubs the frame, you will find you haven't got a spanner and are 10 miles from home