[blueunicorn6]

For 4,500,000,000 years our planet has been rolling around in the cold of space managing to maintain a molten core.

My thermos can’t keep my soup warm until lunch.

[smokingfrog]

You need a new thermos.

[snarkpup]

For 4,500,000,000 years our planet has been rolling around in the cold of space managing to maintain a molten core.

My thermos can’t keep my soup warm until lunch.

Your soup is not heating itself with the decay of radioactive elements. (At least I assume it isn't.)

[Rebelbase]

Get one of those Thermoses that keeps things hot instead of cold.

[norwaypinesavage]

"My thermos can’t keep my soup warm until lunch."

Your thermos has an insulating value (R) of roughly 5. Since it's about a half inch thick, that gives it an R value of roughly 10 per inch. Dirt has an R value of about 1 per inch. The dirt is roughly 1,000,000 inches thick. That gives it a total insulating value of about 1,000,000, compared to your thermos of 5. Earth also has residual radioactivity making up for any loss. That's sort of like putting an electric heater inside your thermos.