Fresnel effect
Every surface turns more mirror-like the more edge-on you view it — which is why a lake glitters near the horizon but is see-through straight down.
Augustin-Jean Fresnel derived the equations in the 1820s. They say reflectance climbs toward 100% at grazing angles: head-on, a water surface reflects only a couple of percent of the light, but viewed across it becomes an almost perfect mirror. The effect is on everything — plastic, wood, skin — and is one of the strongest cues that a rendered surface looks real.
The full equations are expensive, so graphics almost always uses Christophe Schlick's 1994 approximation: a fifth-power curve interpolating from the material's head-on reflectance (F0, about 0.04 for most non-metals) up toward one at the edge. The F factor in a microfacet BRDF is exactly this.