Everything in the universe is made of microscopic atoms that are constantly moving and interact by attraction and repulsion
Feynman said water, air, and rocks look very different but are all made of tiny atoms that pull and push on each other. When it is hot, atoms move faster and bump harder. From this one idea alone, you can explain why ice melts or why smells spread through a room.
Why it matters The same idea underlies chemistry, biology, and the materials that phones are made from.