Q: Why didn’t the string ever win a race?
A: It always tied!
“To be tied” or “to tie” can also mean that no one is winning a game, that the score is the same. It is like in a World Cup Soccer game when the game is tied 1- 1 (sure would be fun to go to Brazil this year, but that has nothing to do with this joke!).
This joke is funny because it plays with the double meaning of the word “tied.”