ELI5 Aircraft Carriers - why are the runways at an angle?

Why do modern carriers feature an angled flight deck instead of a straight full-length runway like their WWII predecessors? How fast does an aircraft have to go to take off from a carrier? Why don't planes just fall off the front of the ship when they take off? What are the costs and challenges associated with building longer runways on aircraft carriers? What advantages do nuclear-powered aircraft carriers have over conventional ones in terms of operational efficiency?  ... 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: hyndis, green_meklar, kal_vas_flam, infernalism, pedopaul, aquatic_mammal, amatulic, generallyawfulhuman. To the 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]