ELI5 Contrails - why do some airplanes leave streaks and others don't?

Does the visible streak an airplane leaves behind depend on the type of plane? Why do some airplanes leave streaks and others don't? What conditions make for good contrails? Why don't jet engines have a grill in front of them to stop birds etc getting sucked in? What are chemtrails?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: runiat, erratic_professional, koleslaw, concise_pirate, thekeffa, lnl_hutz, gnonthgol & zer0summoner 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]