Riddles anyone?

[Madmaxneo]

I am looking for a set of riddles (website?) centered around a theme. Where the theme itself is not to hard but not to easy to figure out. It would be awesome if the theme was an element like, fire, water, air, light or something similar that the party could easily reproduce. I am developing a dungeon where there are 4 primary rooms and each room has a (not to hard) riddle to solve and then there is the final room with the culmination of the riddles. For example if the theme is water then the party might have to pour water into some kind of bowl or basin.

Thanks in advance!