How to explain to my mother-in-law how a thermometer works?

MIL is in your home? Her home? Either way you are not going to be able to reason with her about thermostats or much else. You'll just get exhausted trying. Divert her, fib a little, set the stat where it needs to be, bundle her up or what ever it takes. Put a lock box on the stat if she's messing with it. My dad has dementia and his feet are chronically cold so he wants to keep the house at 85 degrees. I found an electric foot warmer that is basically two big slippers sewn together. He loves it and my poor mom can turn the heat down.

Wouldn't that be nice... if only I could set my thermostat and turn off this incessant cold rainy weather lol!
So if she wants the inside and outside to match you can fib a little, how is she going to know what the outdoor thermometer really says?

I went through this with my cousin. She lost the ability to understand how it worked. To keep the heat off of 85 degrees, I had to install a clear plastic lock box over the thermostat and hide the key. That worked fine. Since she could not fiddle with the thermostat she stopped obsessing about it and was comfortable thereafter at a balmy 74 degrees.

Kathy, ah the thermostat wars... been in the trenches and am still losing the war !!

My Mom was always cold so she would bundle up like the house was 50 degrees, but the thermostat would be 78 degrees. And there Dad would be dressed for the middle of summer as it was too hot in the house for him.

Recently Dad moved to Independent Living, lo and behold, there is the thermostat at 78 again. I think it is a habit ingrained in Dad to get out of bed first thing in the morning and crank up the heat for Mom. But Mom is no longer there. Both the caregiver and I will walk in and ask if we can turn down the temps, waaaay too hot.

I plan to print up a cute sign and place it over the thermostat saying" do not push the temp over 75 degrees, or you will pay extra for the cost" [I know it is a fib, but a therapeutic fib as too hot a house isn't healthy. And Dad is always penny pitching :)

ty for the responses. I am normally brutally honest but have had to make a sticker to go over what the real temp is to it reads "78" even though the actual temp is 72 !! She seems to be falling for it .. for now. I understand with all the chemo and other meds that the poor lady is cold all the time and will agree to the temp being 72 when hubby is sleeping but the second he heads out she flips that thing right bad to 90 !!

I agree with whatever they say or request, then I do what
I think is best.

Yogagirl, finally someone in control!

If you have a weather station, you could bring the outside temperature probe inside and hide it in a secret place. That way the inside and outside temperatures would be pretty much the same.

I hide the real thermostat and a dear dear friend of mine gave me a second one that doesn't work but looks like it does and we have no problems over that thank the lord. All new one over here seem to be removable thankfully

PhoenixDaughter,
Brilliant!

Boy this is a creative bunch, LOL!! Lots of good answers. I just wanted to second FF to say my mom cranks up the heat too. I walk in and am sweating buckets in minutes. She's usually just sitting still while I'm running around like a mad woman working, so our difference in body temps are understandable. But it's still frustrating! I bought her a wonderful clock that has the date and day and inside temp and I always ask her what it is. She's still able to go, "Oh 85 degrees, that's kind of hot, isn't it?" Uhhhh, yes, it is!! :)

This made me remember a Christmas when my father was still alive. The house was so hot that everyone under 80 ended up on the back porch talking. Better to be outside in December than sweating around the Christmas tree.