We’ve all heard the phrase, “Everything is relative.” But is that true when it comes to temperature?

That’s something Marion, Mass. resident Gertraude Heineken has wondered about for years. On hot summer days, after spending time outside, she’ll head back indoors.

"And I come into the air-conditioned house and it is freezing. Siberia inside," explained Heineken.

A glance at her thermostat reveals an indoor temperature of 75 degrees. And yet, in winter...

"Would we ever consider setting the thermostat at 75? I don’t think so. I think it would really be hot," she said. "So what’s going on there? What’s the discrepancy?"

For starters, there are all sorts of ways to cool and heat a house, but no matter how you do it, "the house is like a sponge," explained Lenny Mastropieri, owner of L.M. Heating and Air.

To get — and keep — a sponge-like house at 75 degrees on a hot day, Mastropieri says you need to continuously introduce air that is cooler than 75 degrees. Depending on everything from the size and efficiency of the house to the temperature and humidity outside, that air can sometimes be significantly cooler.

"In the winter time, it’s kinda like the opposite," said Mastropieri. So, that same 75 degrees is being maintained by a steady stream of air that is warmer.

But there is another factor at play here: another thermostat — so to speak — in the truncus encephali, better known as the brain stem.

"How the body regulates temperature and our awareness of temperature are linked but are a little bit separate in how we process it," said Dr. Aaron Young, who teaches physiology and biophysics of at the Boston University School of Medicine.

We humans are homeotherms, which means our body core stays at a precise, consistent temperature — 98.6 degrees. As such, we’ve evolved all sorts of ways to maintain that temperature, no matter the conditions outside. When it’s hot, our body cools our core, and when it’s cold, our body heats it up.

So, how do we do it? In a word: thermoreceptors.

"We have ones that are out on our skin surface, and we call those peripheral thermoreceptors," said Young. "And then we have ones that are located in our brain stem, and we call those central thermoreceptors."

The ones all over our skin surface are single duty. Some receptors sense only hot, and others only cold.

Incidentally, these same thermoreceptors also react to certain chemicals. They are what you feel tingling on your lips when you eat spicy food, for example.

"Or you use mouthwash or you chew minty gum and it feels cool," said Young. "Same thing."

The thermoreceptors on your skin surface send signals to the ones in your brain stem, which regulate how aggressively to either heat or cool the body. When it’s cold out, this is why we shiver — to help heat up our body's core temperature.

Conversely, when it’s hot, we push heat out by moving our blood flow — and with it heat — closer to the skin surface. This is why we might look flushed on a summer day, or when we exercise. We radiate some of this heat out through our skin pores. Convection helps, too: Liquid or air flowing over our hot skin.

"That’s why we feel cooler when you have a fan blowing on you," said Young. "The air is flowing over more quickly and taking heat away."

And then, of course, we sweat.

"And by sweating we put liquid on the skin surface, which is warm to our body temperature," said Young. "And then it evaporates. So we’re losing heat by evaporation."

All this to say that when Gertraude enters her 75-degree house on a hot summer day, her body is in a prime heat-losing state. And, crucially, while her peripheral thermoreceptors feel the cooler air immediately, it takes time for the rest of her system to adjust to the new conditions.

"It’s not like a light switch, you flip it and it immediately comes on," said Young. "There’s a bit of a lag, a bit of a delay."

Chances are if Gertraude waits about 20 minutes for her system to catch up, the house will feel far less like “Siberia” and much more like a spring day. But in the end, it appears she is correct: 75 degrees isn’t always 75 degrees. That discrepancy is not just a figment of her imagination, but it is — at least in part — in her head.