Physics
Particles tunnel through walls
The Sun shines because hydrogen nuclei in its core, too cool to classically clear their mutual electric repulsion, the Coulomb barrier, fuse anyway through quantum tunneling. The odds per collision are vanishingly small, but a star's worth of collisions every second turns that rarity into steady sunlight.
Sources & further reading
HyperPhysics, Georgia State UniversityBarrier PenetrationThe physics definition of how a particle's wave leaks through a barrier.NASA Goddard Space Flight CenterQuantum TunnelingHow a trapped particle can escape a nucleus it doesn't have the energy to leave.WikipediaQuantum tunnellingWhy tunneling is what lets stars like the Sun fuse hydrogen at all.
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