What is a gravitational field and how its strength be measured

A gravitational field is the field generated by a massive body, that extends into the entire space. Every object with mass m experiences a force F when immersed in a gravitational field. The intensity of the force is equal to

where  is the gravitational  constant, M is the mass of the source of the field (e.g. the mass of a planet), and r is the distance between the object and the source of the field. The force is always attractive. 

A possible way to measure the intensity of a gravitational field is by measuring the acceleration a of the object immersed in this field. In fact, for Newton's second law we have:

but since 

we can write

Therefore, by measuring the acceleration of the object, we also measure the intensity of the field.

Learned about life itself

The rocks expand when there hot/warm  and contract when they are cold.