Clouds. Hailstones grow larger as they move about the cloud and collide with rain drops before falling to Earth.

Cell Wall. Cell walls are comprised of specialized sugars called "cellulose".

Vitamin B12. A B12 deficiency in the body cases anemia.

Plastic Rings. The Great Pacific Garbage Patch contains millions of "six-pack" rings, milk jug rings and plastic bags.

Alternating Current. Nikola Tesla invented the alternating-current generator; in 1893 Westinghouse used AC to light the World's Columbian Exposition in Chicago.

Yuri Gagarin. On April 12, 1961, Yuri Gagarin of Russia became the first person in space.