Why Pluto is Not a Part of Our Solar System?

In school, when learning the planets chapter, our teachers told us a sentence using which we remembered the order of all the 9 planets.
My Very Excellent Mother Just Served Us Nine Pizzas.
We learnt it and our minds just got routined that there are 9 planets. But then suddenly, they told us pluto is not a part of our solar system. This 8 planets thing did not get into our heads easily and still today some of us still believe there are 9 planets.
So, is pluto not a planet? After teaching us that pluto is a planet for so many years, why did 'they' told us to forget about it from the planets list? What happened?

The 'Planet' Definition

Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun.
An astronomer from the California Institute of Technology named Mike Brown had discovered a few odd objects beyond all the known planets.

One of those objects, Eris, appeared to be larger than Pluto. At this point, the astronomers from International Astronomical Union (IAU) had two choices.
1. Name all the other found objects as planets
2. Define the word 'planet' in a more deep way
The astronomers selected the second option and gave 'planet' a more deeper definition. According to the new definition, every object including earth should pass three tests to be called as a planet. The tests are as follows :
Test 1 : A planet must orbit the sun
Test 2 : A planet must be round in shape
Test 3 : A planet must clear its neighbourhood of other objects

Testing Saturn

To check this new definition, let us consider the planet of Saturn. Let us test if Saturn is a planet or just an object.

Saturn orbits the sun. Saturn is round in shape. When other objects like asteroids come near to saturn, due to the gravitational pull of the planet, those asteroids change their direction and go away. Saturn passes the three tests in flying colours and so can be called as a planet.

Pluto's Comparison With the Tests

Now let us test Pluto.

Pluto makes one loop around the sun in 248 years. So it successfully orbits the sun and passes the 1st test. Pluto is in perfect round shape. The first two tests are passed. The third test is where pluto fails miserably. In the case of other planets, unlike the asteroids, anything in their(8 planets) path has either been absorbed, captured as moons, or kicked away. After four and a half billion years of jostling for position, the eight classical planets are the last large objects left standing in their orbit. But pluto's case is different.
Instead of being the most distant planet, Pluto seems to be the closest member of the Kuiper Belt, a donut-shaped ring of perhaps a trillion comets and ice balls that orbit beyond Neptune.

At least 200 of these objects are probably big enough to be round, qualifying them as dwarf planets. And like asteroids, they all move in similar ways.

Conclusion

Pluto could not fulfill the new definition for a planet and so it remains being called as a 'dwarf planet'. Having the smallest and the cutest planet, Pluto in our solar system is great. But, 'planet' is a classic word and it has some uniqueness. So Pluto is not part of our solar system and please forget Pluto from the planets list. Now, remember the sentence :
My Very Excellent Mother Just Served Us Nine.

A 3D real time view of Pluto can be observed here.