Why pi is irrational number?

First of all you need to understand how did Pi come up.

consider a cake that you bought and then you cut it in 7(or more) diameters.

Then you take those separate "pastries" and then arrange such that it represents like a crocodile teeth.

(Trying my best to graphically represent it:)

VAVAVAVAV..

Q: Now what is a circumrence?

A: diameter times pi.

the circumfrence(C) of the circle is represented here in dots(..)

....................

:VAVAVAVAV:

so C= p*d (p=pi)

therefore, p=C/d

Hence, Pi is nothing but a ratio (that is a fraction)

And the ratio of the dots above(circumference) to the sum of the V's and A's lengths is never perfect.

Hence it is irrational.