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.
Used left or right, I get to travel over cobblestone or gravel. Used up, I vie for sweet success, used down, I cause men great duress.
Each morning I appear to lie at your feet, all day I follow no matter how fast you run. Yet I nearly perish in the midday sun.
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
What must be in the oven yet can not be baked? Grows in the heat yet shuns the light of day? What sinks in water but rises with air? Looks like skin but is fine as hair?
I have two eyes in the front and a lot of eyes on my tail
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?