There's no question too big and no question too small for WGBH's Curiosity Desk. And this week we turn to listener Louise Erskine from Paxton, Mass., who was hoping to settle a major family debate about a minor household mystery.

"What is the butter dish or butter drawer really for in the refrigerator door? I look online. Nobody seems to know. It's been a raging controversy in Paxton for many years."

Ed Baker, who works the showroom floor at Belcher's Appliance Center in Framingham, Mass., is the one to go to for answers. He's been selling refrigerators for decades and has seen his fair share of butter compartments.

"You’ll find it in almost all the refrigerators — they’ll have a place for the butter," he said. "A little compartment to put it in."

As for why that is, the debate has dragged on in Louise Erskine's family for years.

Theory One: The Brother

"My brother thinks it’s just so that you can find the butter easily and the cover's just so that [the butter] doesn't fly across the room when you open the refrigerator door," said Erskine.

So, what does Ed Baker say?

"It has a function, that’s why they put it in," he explained. "If it didn’t have a function, they’d take it out. The manufacturer doesn’t want to spend any money that he doesn’t have to."

Sure, convenience and butter safety could both be considered functions, but Baker says neither is enough of a function to persuade manufacturers to open up their wallets.

Theory Two: Louise

"I thought that the butter drawer was for keeping the smells from whatever you’ve got in the refrigerator from going into the butter," said Erskine.

Not a bad thought, said Baker. And actually, there’s a chance that the plastic cover does exactly that — to a certain extent.

"That’s a possibility," said Baker. "It’s a closed area, but the thing is not totally airtight."

And even if it does help fend off odors, Baker said that's not the reason these butter compartments have been a ubiquitous feature of the refrigerator door for decades.

Theory Three: The Father

"My father is sure that there is less insulation around the butter drawer and so that if you leave the cover up all the cold air is going out," said Erskine.

So Louise's father's take — essentially — is that the cover is there to keep the cold air in. He’s actually on the right track, sort of. There is, in fact, usually no insulation in this area. But consider how a fridge is cooled.

"The cold air in the refrigerator always comes from the freezer," explained Baker.

That cold air is introduced to the fridge by a fan, usually located in the back near the bottom. Now think about the placement of that butter compartment on most fridges, and the laws of physics.

"The reason they put it on the top is because heat rises," said Baker.

Indeed, the top of the door is usually the warmest area of the fridge. And that plastic cover is designed to double down on that fact. It’s not meant to keep the cold air in — it's meant to keep the cold air out.

"That little cover that goes over it keeps some of the cold from going in there," said Baker. "So, you want the butter to be a little bit softer and it will keep it slightly softer."

Butter becomes easily spreadable at about 60 degrees, far warmer than it is in the fridge. In the 1950s, General Electric introduced a compartment in some refrigerators called the "butter conditioner" — with an active, adjustable heater to keep the butter warm. Not surprisingly, this proved energy inefficient, and they replaced it with that now familiar compartment with the plastic cover, designed to keep the butter just a little bit warmer.

So there’s your answer, Louise. Feel free to spread it around.

More from the Curiosity Desk