Good Riddles

Question: When is a door not a door?

Question: I break down buildings I crumble mountains You can't see me What am I?

Question: What is only a small box but can weigh over a hundred pounds?

Question: Each morning I appear to lie at your feet. All day I will follow no matter how fast you run. Yet I nearly perish in the midday sun. What am I?

Question: I am the black child of a white father, a wingless bird, flying even to the clouds of heaven.
I give birth to tears of mourning in pupils that meet me, even though there is no cause for grief, and at once on my birth I am dissolved into air.

What am I?