Question: What is the center of gravity?

Question: What goes on four legs in the morning, on two legs at noon, and on three legs in the evening?

Question: When you need me, you throw me away. But when you're done with me, you bring me back.

What am I?

Question: When you fall in me your as happy as could be, but you fall into depression when you lose me.

What am I?

Question: Many have heard me, but nobody has seen me, and I will not speak back until spoken to.What am I?