The inlaws showed up yesterday as I was boiling maple sap on a propane burner outside. They started to warn me about not boiling it indoors because it would make my walls sticky and that I would have to wash them as they had to do many years ago. I've been making maple syrup for decades as well, and always finish boiling inside. My parents have always boiled it inside and vented the steam outside. Other than some extra humidity, we've never had sticky walls. I've always been under the impression of basic science that when a solution, like sap, is boiled, only water evaporates, leaving the solutes behind, the maple sugar. I couldn't convince them of this. Am I missing something here?