Well I had to explain it as if it were a kids class. We didn't get into deep theory , constants, equations type stuff. Just overall concepts.

Saying earth surface was round and each country gets different day and night sequences etc, she immediate went to why aren't Australians dangling off if they are at the bottom. So we had to go over center of gravity and gravity itself.

Which led to the Sun and moon being outside the earth. And I had to talk about the immense distances and speed of light concepts. (Since she thought she could get to the sun in about 45 minutes)

Then the questions came about why, if the earth rotates for day and night, aren't people flying off. So I had to go over gravity and inertial laws. Referencing something she knew like being in a car and dropping something, it drops relative to you and doesn't fly back at 100km/h ( inertial frame of reference stuff).

She was obviously skeptical still, especially about stars being distant suns.

And then she came to the conclusion of: how did we get here, if we are tiny spec in the universe on a planet. So we touched on evolution theory.

We eventually got her kids solar system books, and watched some good documentaries on netlix about these subjects and the one about the stages the earth has gone through.

She eventually understood the concepts, but still though we were trying to trick her. Until she went skydiving and realized "the air is 3D" ( her words ). All along she saw the sky as a background 2d backdrop on the INSIDE of the snow globe earth.

I don't know how she missed this concept most of her life, I guess her circle just didn't include people that talked about science stuff, and she took her cues from what she could see...flat ground, sky seems far away, sun is tiny but looks like it hits the mountains and water.