The bell is a percussion instrument and an idiophone. Many types of bells exist.

Two-stroke engine

A two-stroke engine is a type of internal combustion engine with a cycle of only two (power) strokes.

Battle of Thermopylae (480 BC)

The battle of the Greek-Persian War became famous for the heroic sacrifice of the Spartan soldiers.

Types of gears

Two meshing gears transmit torque during their rotational motion.

Viking chieftain (10th century)

Viking leaders were elected from tribe members by the right of blood or an act of bravery.

Quetzalcoatlus, a giant pterosaur

Quetzalcoatlus was one of the largest known flying animals of all time.

Battle of Issus (333 BC)

The battle ended with the overwhelming victory of the Macedonian army over the Persian army led by Darius III.

How does it work? - Optical disc drives

This animation demonstrates the structure and operation of different types of optical disc drives.

Roman gladiators (2nd century)

Gladiators were combatants who entertained audiences in fights against each other or wild animals in ancient Roman arenas.

The phosphorus cycle

The phosphorus cycle describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere.

Oxygen cycle

The oxygen cycle describes the movement of oxygen within its three main reservoirs.

Common European viper

A species of venomous snakes widespread in Europe. Its bite is rarely fatal to humans.

Monsoon wind system

In summer, monsoon winds bring heavy rainfall from the ocean to the continent.

USS Missouri (US, 1944)

The battleship USS Missouri, first deployed during WWII was also deployed in the Gulf War.

How does it work? - Digital camera

This animation demonstrates the structure and operation of digital cameras.


One of the most important monocot crops.

Soil pollution

This animation demonstrates the main sources of soil pollution.

Types of bridges

Types of bridges include beam-, arch-, suspension-, cantilever and truss bridges.

Cities of the world

This animation demonstrates the geographical locations of the Earth's major cities.

Horse chestnut

This animation demonstrates how horse chestnut trees change throughout the seasons

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