ELI5 Vitamin D - what do you mean it's not a vitamin?

How does the body get vitamin D from the sun? Why is vitamin D not actually a vitamin? Why do we need it? What foods are high in vitamin D? What happens if we get too much? Why are there so many types of vitamin B (e.g. B1, B2, B3, B6, B9, etc), but only one type of some other vitamins (such as C and D)?   ... 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: mbisme, mcmustang51, monstrag, gnonthgol, myhandsunclean, mitobear, lasseft, greygreygrey12, vizsla_velcro, splitegg & schnutzel. To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]