Answer:
Runs smoother than any rhyme, loves to fall but cannot climb!
I open wide and tight I shut, Sharp am I and paper-cut fingers too, so do take care, I'm good and bad, so best beware.
In the night a mountain, in the morning a meadow.
When young, I am sweet in the sun. When middle-aged, I make you gay. When old, I am valued more than ever.
The more of it there is, the less you see.
When is it bad luck to see a black cat?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?