How Come?: Why is it difficult to move underwater?

Walking across a pool can feel like trying to walk in a dream, each step seemingly mired in quicksand. And running in water may be the closest thing we have to real-life slow motion. As we laboriously jog across the pool to our friends at the edge, it seems like our voices should change, too, like in those slow-mo tv clips: "I'lll beeee riiight theerrrre."

So, yes, it can be a drag to walk (or wave arms) underwater. As it turns out, "drag" is precisely the problem. Wading through water is very different than striding through thin air.

It all comes down to density - how much mass is crammed into a given volume. Take iron. It's obvious that iron's molecules are jam-packed; it's much denser than wood. You can easily pound a nail into plywood, but just try hammering a nail into an iron frying pan.

Likewise, liquid water is much denser than air. How much? The water in a swimming pool at sea level is nearly 800 times as dense as the air above the water's surface. And here's where the drag come in. When you walk or run on land, you don't feel the small dragging force your body experiences as it moves through air. (A fast-moving jet or space shuttle, on the other hand, experiences significant drag as it glides through Earth's atmosphere.)

But the resistance you feel submerged in water is about 800 times the wind resistance you feel out of the pool. And the faster you try to move, the greater the drag you'll feel.

In addition, the wider and bulkier the object, the more drag it experiences as it forces its way through a wall of water. The rounded, streamlined shape of fish, dolphins, and submarines reduces drag in water. And for us, streamlining and narrowing our own shape by swimming rather than walking reduces the drag we feel, too.

But even if we're not swimming, moving through water can actually be easier than moving on land. The encompassing pressure of the water provides a buoyant force that opposes the downward pull of gravity. The result: In water, your body weight is reduced by up to 90 percent.

Water's weight-easing cushioning coupled with its high resistance to motion makes it ideal for low-impact exercise and rehabilitation after injury. People with balance problems can exercise without worries of falling; those with joint problems can exercise without the stress of walking or running on land. Stretching can be easier underwater, too.

Meanwhile, water's built-in resistance means we can work muscles and raise our heart rate simply by moving vigorously, no gym equipment required. (However, we still need to do weight-bearing exercise on land to strengthen bones.)

Some companies have even developed underwater treadmills for use by human athletes, dogs and horses.