Flow.

Have you ever been occupied with something feeling that time and the world around you seems to stop existing?
It typically happens when the task isn't to easy and not to difficult. You have "a kind of perfect challenge."
The psycholog Mihaly Csikszentmihalyi is known for describing this fenomen calling it flow. He even states flow might be the secret to happiness. I do not entirely agree with him because happiness in my opinion can be achieved outside the state of flow. But a high degree of flow in your life does probably make you more likely to be happy and state: It's a wonderful world!

This is a short lecture where Mihaly Csikszentmihaly is talking about flow:

Mihaly Csikszentmihalyi on flow.