How Come?: Lesson in anti-gravity at your fingertip

How come when you put your finger on a straw filled with liquid and pick it up, the liquid stays in the straw rather than dripping out? asks reader Kaiylah Watts.

Fun while waiting in a restaurant: the straw trick. Put a finger over the end of your straw before lifting it from your beverage, and it's like you've invented your own anti-gravity device. The liquid, amazingly, stays suspended in the straw rather than pouring out onto the table. But lift your finger and well . . . oops.

How does one little finger make water defy gravity? As it turns out, the power isn't exactly in your finger. It's in something virtually invisible: the air around us.

At close range, in a quiet room, air is invisible and unfelt. Only when air currents move against us (the wind), or when we look at the blue sky (the mass of air from a distance reveals itself to be light blue), do we feel and "see" the air. But at sea level, the Earth's atmosphere is pressing on each square inch of us (and of everything else) with about 14.7 pounds of pressure.

Have you ever picked up a 15-pound dumbbell or three 5-pound bags of potatoes? Then you get a sense of how surprisingly weighty Earth's air is, pushing with nearly 15 pounds of force on each postage-stamp-sized spot on your body.

Of course, the air also exerts its pressure on your lovely restaurant beverage, including the liquid inside the straw. But sealing the open end with your finger changes everything.

How? When you cover the top with your finger and lift the filled straw, the liquid inside begins to fall downward. At first, you'll lose a few drops. But as the liquid sags down into the straw, the air trapped in the space between the liquid and your finger spreads out, becoming less dense. That means less pressure on the liquid remaining in the straw.

Since the pressure from above has decreased, it's now lower than the pressure from the air at the straw's bottom. With the pressures unbalanced, the liquid experiences a greater upward force from below. In fact, the force produced by this pressure imbalance is strong enough to hold up the weight of the liquid, offsetting the force of gravity.

Here's another experiment to experience air's weighty presence. Take an empty coffee can; punch three holes in the bottom and one in the plastic lid. In the sink, fill the can just halfway with water. Replace the lid. Hold the can and press your finger over the top hole, while pushing down on the lid. Water should flow from the bottom of the can.

Now, slowly let up the pressure on the lid hole (but keep it covered). The stream of water should slow, then stop. Why? By pressing on the lid, you pressed on the mass of air inside, which shoved the water out of the can. When you stopped pushing, the pressure of the air outside the can pushed the water up from the bottom.