A flea is a tiny insect whose body length ranges from a mere 2 to 4 millimeters but can jump a height of up to 100 times that length.
When such a flea was confined to a glass box, at first the flea jumped and hit the glass above it. Later on, the flea began jumping just high enough so that it wouldn't hit the glass above, and even after the glass box was removed, the flea was unable to jump over the height of the glass box. Without realizing that it could freely jump and jump even higher, the flea was unable to exhibit its ability.
The same applies to us as well. When we set for ourselves a limit to our ability, we are unable to overcome that limit and achieve something far greater. Yet, when we fix our eyes on the almighty God and rely on Him, we can do anything (Mark 9:23).