What is ‘water memory’? why this homeopathy claim doesn’t hold water

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The U.S. Food and Drug Administration announced Dec. 18 that it plans to crack down on dangerous or dishonestly advertised homeopathic products — a class of products that sellers claim treat diseases by delivering extremely diluted traces of the substances that cause those diseases in the first place. If certain homeopathic remedies become more difficult to access due to the crackdown, what will homeopathy users miss out on?
Homeopathy dates to the 1700s, according to a statement from the FDA, and relies on the idea of “like cures like” — that symptom-causing chemicals can, at low enough doses when mixed with water, treat the symptoms that those substances cause. In other words, a chemical that causes vomiting would be given at a very diluted concentration to treat vomiting. And the more diluted the substance, the more potent the beneficial effects, the thinking goes. [11 Surprising Facts About the Immune System]
The British Homeopathic Association (BHA)’s website acknowledges that homeopathic remedies might seem “implausible for many people,” because “the medicines are often — though by no means always — diluted to the point where there may be no molecules of original substance left.”
BHA offers two partial explanations — both commonly expressed by homeopathy advocates — for why homeopathic remedies might nonetheless have benefits for people who take them. The first is that a homeopathic substance, even diluted to the point that it no longer can be detected by even the finest instruments in a sample of water, changes the structure of the hydrogen bonds in the water. Homeopaths term this supposed effect “water memory.”
Hydrogen bonds are real attractions between the hydrogen atoms in one water molecule and the oxygen atoms in that molecule’s neighbors. (A water molecule has two hydrogen atoms and one oxygen atom.) These bonds account for many strange features of water, including the crystalline structure of ice that causes it to expand and float on top of liquid water. But May Nyman, a chemistry professor at Oregon State University, told Live Science that the whole idea of “water memory” doesn’t make sense.
“I don’t believe in water memory, because water molecules move. They’re constantly rotating with respect to each other, forming hydrogen bonds, breaking hydrogen bonds,” Nyman said. In other words, there’s no structure in a solution of liquid water permanent enough to account for any long-term memory, she said.