ELI5 Tennis - why is it scored 15, 30, 40?

Why is tennis scored so oddly? Why do pro tennis players always take 3-4 balls and then discard some immediately before serving? Why do tennis ball cans have a soda lid on them? How does hawk-eye work?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: lo0rdAri, wanikuma, capnaspen, carlr818, keklenny06, woda8566 and th3_gruff To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]