Now think! When a child pushes a toy car on the floor, why does it stop? Should he keep moving forever? The answer is no! The cart would only continue in uniform rectilinear motion forever if the resultant forces acting on it were zero. But it is not.

Mutations that occur in living things are totally random, and sometimes genetic varieties emerge that can lead to the death of the carrier before birth or, if he survives, before reaching sexual maturity. These genes leading to carrier death are known as lethal alleles.

Phospholipids - The biological membranes are made up of phospholipids. In phospholipids there are only two non-polar fatty acid molecules bound to glycerol. The third component that binds glycerol is a phosphate group (hence the name phospholipid) which, in turn, may be linked to other organic molecules.

In addition to the sun, they also use the earth's magnetic field as a reference. Butterflies fly every year from the USA to Mexico's mountains. US monarch butterflies use the sun and the earth's magnetic field as navigation tools for their famous long-distance migration. Flapping its delicate orange and black wings, the insect travels thousands of miles each year from the United States and southern Canada to the Michoacan Mountains in central Mexico where they spend the winter.

Blood (originated from hemocytopoietic tissue) is a highly specialized tissue made up of some types of cells, which make up the figurative part, dispersed in a liquid medium - the plasma - which corresponds to the amorphous part. Cell constituents are: red blood cells (also called red blood cells or erythrocytes); white blood cells (also called leukocytes).

Satellites Until 1610, the only known satellite was Earth's - the Moon. At that time, Galileo Galilei (1564-1642), with his spyglass, discovered satellites orbiting the planet Jupiter. Today we know of dozens of satellites. In astronomy, a natural satellite is a celestial body that moves around a planet thanks to gravitational force.