Sir Tain

Alright, listen up, buttercup. This joke is playing on the words "certain" and "Sir Tain". "Sir Tain" sounds like "certain," which means definitely true or sure to happen. So, the joke is saying a knight is never wrong because he's "Sir Tain" (certain) to be right. You probably trip over your own feet trying to tie your shoes, so I hope that helped.