He stressed that life needs four elements to evolve: hydrogen, oxygen, nitrogen, and carbon.

Chemically, only carbon can form the extra-strong double bonds with other elements needed to make certain complex molecules, the building blocks of life.

Sandman

Wald refuted the claim that silicon, not carbon, could be the basis of other life. Silicon cannot form chemical double bonds, and therefore, forms crystalline chains instead of discreet molecules.

"When you're finished, you've got a rock," said Wald. "But if you want to make living organisms, you have to use carbon.

But Wald called water the most important molecule of life. And if water didn't break all the rules of chemistry, life could not exist.

Most solids are denser than their corresponding liquids; for example, an iron nail sinks to the bottom of a cauldron of molten iron.

But ice is actually less dense than liquid water; an ice cube floats in a drink. "Nothing else does that--it's just crazy," said Wald.

Therefore, when a pond freezes, the ice forms an insulating layer on the top, and the pond does not freeze all the way through.

If ice were "normal," it would sink and ponds would freeze from the bottom up and ponds would turn to solid ice.

Come spring, these icebergs would not melt completely. The warm, watery environment in which life evolved would not have existed.

Fortunately for life, water can be found throughout the universe, Wald added.