Special & general relativity — time dilation, length contraction, simultaneity, spacetime and gravity as curved geometry.
Emit two light pulses in opposite directions and transform the events into a moving observer's frame to see both frames measure exactly c.
Follow a photon bouncing between spacecraft mirrors and compare laboratory and spacecraft clock readings as velocity rises.
Compare a rod's rest length with its laboratory measurement while it moves, using synchronized endpoint gates.
Flash both ends of a moving train at once in the station frame and see why the train frame disagrees.
Send one twin to a distant marker and back and compare the ages accumulated along each worldline.
Release two opposed radiation packets from a resting source and track energy exported against rest mass lost.
Combine a carrier velocity with a probe launched from it and compare the relativistic and classical sums.
Place an event on a Minkowski diagram, tilt the moving-frame axes and classify the interval as timelike, lightlike or spacelike.
Explore Schwarzschild spatial geometry with an orbiting clock and compare coordinate radius with proper radial distance.
Place clocks at two radii near a compact object and compare tick rates and the redshift of light sent upward.