A Harvard Natural Sciences lecture recently demonstrated a microscale version of the classic thermite reaction using two rusty iron spheres, each about 2 kg and 7 cm across. One ball is wrapped in a single layer of aluminium foil; a glancing strike ignites the mixture, producing a loud crack, bright yellow sparks and heat. The setup fits on a lab bench and costs under $20 in materials.

The underlying chemistry mirrors the textbook equation 2 Al + Fe₂O₃ → Al₂O₃ + 2 Fe, releasing ‑849 kJ mol⁻¹ and driving temperatures near 2200 °C—hot enough to melt iron (melting point 1530 °C). Because the reaction is self‑sustaining, no external oxygen is required, which is why thermite finds use in rail‑track welding and underwater repairs. Safety goggles, a lab coat and careful positioning are mandatory during the demo.

Students leave the session with reusable iron balls; repeated strikes merely regenerate sparks as long as fresh rust remains. If the oxide layer fades, soaking the spheres in salt water and air‑drying restores reactivity, a tip first shared by Larry Peck of Texas A&M. The demonstration packs visual impact into a safe, repeatable format, making microscale thermite a staple for chemistry outreach.