"Professor Snape, can potions be diluted?"

Her class froze, she persevered.

"If you add water to this sleeping potion would the ingredients react? Or dilute?"

"...I am not here to answer inane questions," he drawled, "next time do some research before bothering me with your flights of fancy."

"Right," a breath, "and, what would you suggest I look into?"

One hour later saw her flipping through a copy of Puer Potio Ingredientia, trying to commit various reactions to memory. She was rather confident she could come up with one decent question by tonight's detention.

...did potions have an expiration date?