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A spark from a handheld firework flies off at a scorching 2,000∘C2{,}000^\circ\text{C}, yet when it lands on your skin, it barely stings. Meanwhile, falling into a bathtub of water at just 50∘C50^\circ\text{C} would cause severe burns across your entire body.
How can something with a much lower temperature deliver vastly more destructive energy? To solve this puzzle, we have to look down at the individual atoms.

The Kinetic Theory of Matter

All matter is made of tiny particles (atoms or molecules) in constant, random motion. The kinetic theory of matter states that the physical properties of a substance arise entirely from the motion and collisions of these microscopic particles.
Since every moving particle carries kinetic energy, what does a standard thermometer actually measure?

Temperature: Average Particle Speed

Temperature is a measure of the average translational kinetic energy per particle in a substance. In the late 1800s, Ludwig Boltzmann and James Clerk Maxwell formulated statistical mechanics to prove that macroscopic temperature scales directly with this microscopic average.
For an ideal monatomic gas, the average kinetic energy ⟨K⟩\langle K \rangle of a single particle relates to the absolute temperature TT in Kelvin by the equation:
⟨K⟩=12mvrms2=32kBT\langle K \rangle = \frac{1}{2} m v_{\text{rms}}^2 = \frac{3}{2} k_B T
Here, kBβ‰ˆ1.38Γ—10βˆ’23Β J/Kk_B \approx 1.38 \times 10^{-23}\text{ J/K} is the Boltzmann constant (the conversion factor linking microscopic energy to macroscopic temperature), and vrmsv_{\text{rms}} is the root-mean-square speed of the particles.
Because kinetic energy is directly proportional to temperature, there is a fundamental lower limit to how cold matter can get.