ELI5 Hawk Eye in Sports - how does ball tracking work?

How does the technology work to determine if a tennis ball is in or out? How accurate is it? Why is it difficult to replace all sports officials with systems like hawk eye?   ... 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]