Five-year-old Ally was watering the flowers for her mom with a garden hose one summer day. She put her thumb over the end of the hose to see if she could stop the water from coming out. At first it was easy, but it quickly got harder and harder to hold back the water. Even when she clamped her other hand over her thumb, she couldn't keep the water from spraying out.

We've all tried this little experiment at one time or another. And we know that no matter how hard we push against the flow, the water will eventually win out. Why?