### Theory:

In the olden days, people travelled long distances only on foot. Animals were the only mode of land transport to carry heavy goods from one place to another. People also used boats for transportation as they knew that the logs would float on water. Only after the invention of wheels, transportation took a drastic change.

In the 19th century, the invention of the  steam engine led to the development of railroads to carry passengers and goods. Apart from these, automobiles such as motor cars, buses and trucks were introduced. Meanwhile, air transports developed to reduce travel time.

But, to choose the appropriate mode of transport to reach the destined destination, we need to calculate the distance and time taken to travel. And, hence arises the question on the measurement. Measurement is used everywhere around us.
Example:
Stitching clothes, cooking food, farming, making a cupboard, etc.

The comparison of unknown quantities (a number) with some known fixed quantities (unit) is known as measurement.
For instance, if the length of a book is found to be twice the length of your hand, then $$2$$ is the number and 'hand span' is the unit selected for the measurement. For the unit of 'foot span', the number value may differ.
A hand span is an outstretched distance between the tip of the thumb to the tip of the little finger, whereas a foot span refers to the length of one's foot.

But, this does not give the right measurement of the book since the length of one person's hand or foot will not be the same as the other person. You cannot correctly measure the actual length of the book unless you know the exact measurement of your hand or foot.Thus, we need a standard unit of measurement that does not vary from person to person.
Reference:
https://4.bp.blogspot.com/-TUP4CthKkLA/UgEIRfO0Q7I/AAAAAAAAAaA/DJiSc6t1cJM/s1600/4649-Bullock+Cart.jpg