Time. There never seems to be enough of it. Sometimes we want more hours in the day and sometimes we wish the day would fly by (like on Fridays so we can get to the weekend). Children have no concept of time; they have to learn it. Maybe that's another way in which we should have faith like a child. God doesn't work on our time-frame (chronos) and we have no concept of His (eternal).
"But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day."
(2 Peter 3:8 NIV)