ITT we post Pokémon or moves where we don't understand why they are the type(s) they are and ideally help each other to "get" it.

I'll kick things off with pic related. Dragon I sort of understand based on their designs, but I really don't get Dialga and Palkia's secondary types at all. I mean, maybe at a stretch Dialga is Steel-type because metal endures the ravages of time relatively well or because clocks and other timekeeping instruments tend to be metallic, but for the life of me I can't figure out why Palkia is Water-type. Also, is there any reason at all why their respective signature moves are both Dragon-type beyond them needing to get STAB on them?