Britannica Student Encyclopedia - 2010

of the most common causes of waterfalls.

Flowing water wears away the rock

in a riverbed. Riverbeds sometimes contain

areas of hard rock and areas of soft

rock. In these cases the river wears away

the soft rock before the hard rock. The

hard rock that is left creates a steep wall.

The river water plunges over the rock

wall, forming a waterfall.

Waterfalls also form as a result of movements

in Earth’s crust. These movements

can cause huge blocks of rock to

rise or drop. This creates rock walls over

which rivers may fall. Large, moving

sheets of ice called glaciers also can cut

away rock to form waterfalls.

#More to explore

River • Rock

Water Lily

Water lilies are plants that grow in still

or slowly moving water. They like

ponds, streams, and the edges of lakes in

tropical and mild areas. Their floating

leaves are often called lily pads. Frogs

like to sit on them.

Havasu Falls is a scenic waterfall in the

Grand Canyon.

Niagara Falls

is the bestknown

waterfall

in North

America. It is

partly in

Canada and

partly in the

United States.

24 Waterfall BRITANNICA STUDENT ENCYCLOPEDIA

 

Water lilies grow from the muddy bottom

of a body of water. Thick underwater

stems are buried in the mud. Long

stalks reach up from the stems to support

the round, waxy leaves. Some leaves

grow to 6 feet (2 meters) across. The

leaves usually float on the surface, but

sometimes they are underwater.

A single flower blooms at the end of

each stalk. The flowers are shaped like

stars or cups. They can be white, cream,

pink, yellow, red, orange, purple, or

blue. Some flowers open only in the

morning. Others open only in the

evening. For example, the Egyptian

lotus has white flowers that bloom in the

night and stay open only until midday.

Water lily plants also produce a fruit. It

looks like a nut or a berry. Inside are

seeds. When the fruit opens the seeds

sink or float away to produce new

plants.

Water lilies provide shade and hiding

places for fish and other water animals.

Fish, ducks, and other animals eat them,

too. Some people grow water lilies in

garden ponds.

#More to explore

Frog • Plant

Weapon

A weapon is an object used to harm or

kill living creatures or to destroy property.

Individual people and armed forces

use weapons to defend themselves or to

attack an enemy. People also use weapons

for hunting.

EarlyWeapons

The first weapons were things such as

stones and wooden clubs. Early peoples

used such objects to protect themselves

and to hunt for food. Later, ancient

people made weapons out of bronze

and iron. They developed special

swords, spears, and bows and arrows for

warfare.

Ancient people also invented special

weapons to get past the thick walls of

cities and castles. Catapults were weapons

that hurled objects over the walls.

Battering rams were large wooden beams

that could break down doors and walls.

During the Middle Ages (about AD 500

to 1500) Europeans began to use more

complex and deadlier weapons. By the

900s they were using powerful crossbows

that released arrows with a trigger.

In the 1300s they developed large guns

called cannons. These weapons were

powered by gunpowder, an explosive

mixture of chemical substances. (The

Chinese had developed gunpowder hun-

Water lilies bloom in the water. The plant’s

leaves float on top of the water.

Poisonous

chemicals and

deadly germs

can be made

into weapons

called

chemical and

biological

weapons.

BRITANNICA STUDENT ENCYCLOPEDIA Weapon 25

 

dreds of years before.) In a cannon,

exploding gunpowder shot a heavy stone

or metal ball out of a tube.

ModernWeapons

In the mid-1400s the Spanish invented

the first gun that could be fired from a

man’s shoulder. It was called a harquebus.

A larger shoulder gun, called a musket,

replaced the harquebus in the

1500s. The first pistols, or handheld

guns, also came into use at this time.

Rifles came into wide use in the 1800s.

Bullets shot from a rifle spin around as

they travel. This makes shots fired from

rifles very accurate. In the mid-1800s

the first successful machine guns also

began to be made. Machine guns could

quickly fire many bullets in a row.

Armed forces used many deadly new

weapons duringWorldWar I (1914–18)

and WorldWar II (1939–45). Some of

these included tanks, poison gas,

flamethrowers, and missiles (rockets that

carry explosives to a target). Airplanes

also played a major role in these wars

because they could drop explosive weapons

called bombs.

The deadliest weapon ever used was the

atomic bomb, a kind of nuclear weapon.

WorldWar II ended after the United

States dropped two atomic bombs on

Japan. More than 100,000 Japanese

people were killed.

Weapons Today

Today armed forces continue to use

guns, missiles, bombs, and other modern

weapons. Police officers in many

countries carry clubs, handguns, rifles,

or stun guns. (Stun guns use electrical

shocks to stop people from moving.)

#More to explore

Army • Bomb • Nuclear Energy • Police

• Rocket •War

Weasel

Weasels are meat-eating mammals that

are excellent hunters. They are closely

related to mink, ferrets, and wolverines.

The crossbow was a powerful weapon that

could shoot a bolt (short arrow) as far as