Y. Kasamatsu

A proton can't be seen as a simple ball. If you hit it with a photon, it can absorb energy and jump into an excited state, like a bell that starts ringing at a certain frequency after being struck. These states are called nucleon resonances, and they live very briefly before decaying into other particles.

An atomic nucleus usually follows its own rules: its decay doesn't care about temperature, pressure, or molecular neighbors. But the uranium-235m isotope is a striking exception. This nucleus sits in an excited state, like a tightly coiled spring, and sooner or later it releases the extra energy.