So a few feet underwater, six miles off the north coast of Oahu, I feel a tap on my shoulder. A torpedo-shaped Galapagos shark coasts by. I come up for air.

"Your knee was outside the shark cage," sez my responsible pal.

This is good to know.

"Shut up," I tell him, just as you would.

In the bobbing cage, sucking a snorkel tube, circling sharks ripple in sunlight, all IMAX, Discovery Channel and light show. It is an irresistible swim of beautiful greys and blues.

Fools try to touch the sharks, a hypnotic arm's length away. This is why God gave us liability waivers.

Never give them a knee.