Answer:
A time when they're green. A time when they're brown. But both of these times, cause me to frown. But just in between, for a very short while. They're perfect and yellow. And cause me to smile.
What flies around all day but never goes anywhere?
What do you throw out to use and take in when you're done?
What is often returned, but never borrowed/
Hard to catch, easy to hold. Can't be seen, unless it's cold
Dry though I may be, I am still a sea.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?