This frog hides its own blood to turn invisible while it sleeps

Glass frogs have translucent skin on their undersides, revealing their hearts, livers and coiled intestines beneath. While resting during the day, they pack roughly 89% of their red blood cells into their liver, which acts like a mirrored storage container, making the frog up to 61% more see-through and far harder for predators to spot against a leaf.

— Wikipedia contributors, Centrolenidae — Glass frog