Original riddle (checked): What opens up like a flower to keep you dry in the rain?
Grammar check: The sentence is grammatically correct. It is a clear, complete question. If you want a slightly simpler version, you could say: What opens like a flower and keeps you dry in the rain?
Hint: You hold it above your head when it rains.
Answer: An umbrella.
Why: An umbrella opens wide like a flower and covers you so the raindrops fall on it instead of on you.