The old texts on this site began the subject with a joke. Take two equal numbers, $a=b$, and do some innocent-looking algebra: $$\begin{aligned} a&=b\\ a^2&=ab\\ a^2-b^2&=ab-b^2\\ (a+b)(a-b)&=b(a-b)\\ a+b&=b\\ 2b&=b\\ 2&=1. \end{aligned}$$Where is the mistake? Going from the fourth line to the fifth, we divided both sides by $a-b$. But $a=b$, so $a-b=0$, and we divided by zero. The script hides the crime: $a-b$ looks like any number, and only someone who remembers the first line notices that it is zero. A disguised division by zero is the simplest form of what mathematicians call an indeterminate form, the famous $\frac00$.