Water can exist in three states (or three phases):

• Solid phase:  The particles in a solid are strongly bonded to one another. Ice cubes maintain their form regardless of the container that holds them.

• Liquid phase: The particles are no longer in an ordered state. The bonds between molecules are broken, and the liquid water takes the shape of its container.  The particles are very close to one another, and so a liquid is incompressible.

• Gaseous phase: Agitation and disorder are at the maximum level. Water vapor occupies all of the space in a container. The distances between molecules are large. A gas in compressible.

Note that water vapor is invisible.

At home or at school, here is the ideal interactive tool to review your arithmetic tables.

The principal agents of erosion of the littoral remain waves, tidal effects and marine currents. The masses of moving water act like "bumper collisons" that alter the rock.

One should not underestimate the importance of chemical degradation of the rock by spray (causing dissolution of material), as is the case for calcereous rock.

The form of the littoral depends on a great many parameters. It could be  something like simple beaches composed of sand or pebble beaches, or it could be a matter of cliffs that have been fashioned by successive rockfalls.

Frozen water occupies a volume 10% greater than when it is in its liquid state.

For this simple reason, water that infiltrates faults or fissures is able to break even the hardest rock during  several cycles of freeze-thaw.  Rocks that have been damaged in this way display breakages that are clean and straight.

This type of degradation of rock is common in mountain regions, where the temperature oscillates regularly around 0° C.

The U-shaped valley is chraracteristic of glacial erosion. The passage of the   gigantic mass of a glacier marks the landscape with imposing tracks. Its abrasive power tears away the walls of blocks of rock. These are crushed and carried downstream.  These are the moraines.

Glaciers are numerous during glacial periods (the last one ended 10,000 years ago). Numerous lakes and fjords bear witness to their presence, long after they have receded.

Thick  sedimentary layers that have accumulated at the bottoms of fjordsand lakes conceal the true depth of the original valley.

The V-shaped valley is typical of one that has been carved by flowing water. The erosion is more pronounced when the water flow is a heavy one, and the water carries suspended particles (sedimentary load).

Sediments torn away from the walls are carried away by the water, downstream, where the speed of the flow is too weak to carry them any further.

The respiratory movements (inspiration and expiration) enable the renewal of the air in the lungs.

This animation represents the Japanese abacus (soroban). Balls are strung along rods (columns). Each rod corresponds, going from right to left, to ones, tens, hundreds, thousands…

The upper balls have a value of 5. The lower ones have a value of 1. In other words, an upper ball on the third rod from the right has a value of 5 hundreds (500).

A lower ball on the second rod from the right has a value of ten (10).

A ball is only counted when it has been moved  down to the transverse (horizontal) bar.

The principle of the abacus has been known since the 3^rd millenium BCE (the Mesopotamians). It enables one to carry out simple operations like additions and subtractions, but, in expert hands, the abacus can also be used to carry out multiplications, divisions, and even the calculation of roots.

Etymologically speaking, the word metamorphosis comes from the Greek  “meta-morphosis”, which referred to any change of form, or transformation.

This word applies   perfectly to the fascinating transformation undergone by certain animals in order to pass from their larval to their adult stage.

This animation illustrates the principal stages in the metamorphosis of a butterfly – the Machaon.

Here is a geometric method that enables you to find the center of a circle.

The principle rests on the following property: a right triangle inscribed in a circle has for its hypotenuse the diameter of the circle.