I can bring tears to your eyes; resurrect the dead, make you smile, and reverse time. I form in an instant but last a lifetime. What am I?

What has no hands but might knock on your door, and if it does you better open up?

What is mostly reliable but you won't make a peep unless it goes beep?

I stump people, I hurt people's brains. But when people get me, when they understand me, they are satisfied. What am I?

A nightmare for some, for others a savior I come, my hands cold and bleak, its the warm hearts they seek. What am I?

What causes laughter at times but always causes depression? The victim might even try to laugh it off.