The cracking sound made by knuckles, necks, backs and other joints when they're cracked is the sound of bubbles popping in the joints fluid.
Sweat doesn't smell bad. Body odour is caused when the skin bacteria feed on sweat.
Just one drop of blood contains about 10,000 white blood cells and 250.000 platelets.
Heterochromia is a condition in which people have different colour eyes.
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From @farin007