The Martians are made of sugar.
Later, same day. Kroger said that the Martian metabolism must be like Terran (Earth-type) metabolism, only with no pancreas to make insulin. They store their energy on the outside of their bodies, in the form of scales. He's watched them more closely and seen that they have long rubbery tubes for tongues, and that they now and then suck up water from the stream while they're watching us, being careful not to get their lips (all sugar, of course) wet. He guesses that their "blood" must be almost pure water, and that it washes away (from the inside, of course) the sugar they need for energy.
I asked him where the sugar came from, and he said probably their bodies isolated carbon from something (he thought it might be the moss) and combined it with the hydrogen and oxygen in the water (even I knew the formula for water) to make sugar, a common carbohydrate.
Like plants, on Earth, he said. Except, instead of using special cells on leaves to form carbohydrates with the help of sunpower, as Earth plants do in photosynthesis (Kroger spelled that word for me), they used the shape of the scales like prisms, to isolate the spectra (another Kroger word) necessary to form the sugar.
"I don't get it," I said politely, when he'd finished his spiel.
"Simple," he said, as though he were addressing me by name. "They have a twofold reason to fear water. One: by complete solvency in that medium, they lose all energy and die. Two: even partial sprinkling alters the shape of the scales, and they are unable to use sunpower to form more sugar, and still die, if a bit slower."
"Oh," I said, taking it down verbatim. "So now what do we do?"
"We remove our boots," said Kroger, sitting on the ground and doing so, "and then we cross this stream, fill the boots with water, and spray our way to freedom."
"Which tunnel do we take?" asked Pat, his eyes aglow at the thought of escape.