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When blood is exposed to air, it clots. Heparin allows the blood to come in direct contact with air without clotting.
After a vasectomy, it takes about 12 ejaculations to clear out sperm that were already beyond the blocked area.
A good example of polar molecules can be understood when trying to make a cake. If water and oil are required, they will not mix together. If you put them into a measuring cup, the oil will rise to the top while the water remains on the bottom.
Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.
Since 1988, the CDC has reported a 99% reduction in bacterial meningitis caused by Haemophilus influenzae, due to the introduction of the vaccine against it.