Time Dilation
A clock moving past you ticks slower than your own, by the Lorentz factor — negligible at human speeds, enormous near light speed.[1] This isn't a trick of measurement; it's real, and we see it constantly. Muons created high in the atmosphere should decay long before reaching the ground, yet they arrive — their internal "clocks" slowed by their speed. Atomic clocks flown around the world return measurably behind their twins on the ground, and GPS satellites must correct for time dilation or navigation would drift kilometres off within a day.[2]
At walking pace γ is 1.0000000 — relativity hides. But near the speed of light it soars: at 87% of c a moving clock runs at half speed, and it climbs toward infinity as v → c, which is why nothing with mass can reach it.