Scientific Explanation
Joule proved the equivalence of mechanical work and heat through precise experiments. Count Rumford showed heat cannot be a finite "substance" since boring cannon barrels produces unlimited heat.

The gym-bro consultant who believed heat was a substance
A 30-something gym-bro consultant. Claimed heat was a liquid-like substance (caloric) that flows from hot to cold objects. Obnoxiously tries to pour "caloric energy" into others. Joule's experiments revealed heat's true nature as molecular kinetic energy, denying his existence as a substance.
Proposed by Lavoisier and others. Denied by Joule's measurement of the mechanical equivalent of heat.
Joule proved the equivalence of mechanical work and heat through precise experiments. Count Rumford showed heat cannot be a finite "substance" since boring cannon barrels produces unlimited heat.
"Intuitively understandable explanations" are not always correct. The metaphor of heat flowing like liquid is useful but not reality.
Catchphrase“I'll inject you with caloric!”
Hydrogen's combustion energy symbolizes "molecular motion." A witness to caloric theory's denial.
“熱は液体だ!高いところから低いところへ流れる!”
“俺を注入すれば、どんなものも熱くなる!”
“分子の運動?そんな目に見えないものを信じるのか?”
“ランフォード伯の砲身削り実験は…見なかったことにする”
“エネルギーは保存される?なら俺も保存してくれ!”
Hey! Everyone got enough caloric? I'll inject some for you!
...What? I'm not a substance? A concept? ...No, you'd feel heat if you touch me!
Caloric: "Hydrogen! When you burn, it's ME being released!"
Hydrogen: "No. Energy is released when hydrogen-oxygen bonds break and new bonds (H₂O) form."
Caloric: "That means... my role is..."
Hydrogen: "Energy only changes form, never created or destroyed. That's the First Law of Thermodynamics."
...Even if not a "substance," heat truly exists. Only its form changed.
My name lives on as "calorie." ...Just as a unit, though.