ELI5 El Niño and La Niña - what exactly are they?

When did people start noticing El Niño and La Niña? What exactly are they? Where does the name come from? How do they affect global weather patterns? Do they really affect the Earth’s rotation? ... 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: impar-exspiravit, thefiercedeity, nianp, colin52 and genxcub To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]