He understood, the thing is that the square root of a negative number is only impossible if you exclude "imaginary" numbers as a solution.

Very much like saying 3-6 is impossible if you don't think negative numbers are possible.

Just like banks might need negative numbers to say you're in debt, imaginary numbers are useful for other things, engineers use it to describe electricity in AC circuits for example

So you can say that imaginary numbers aren't possible, but that excludes real solutions that help you solve problems, very much like excluding fractional numbers would stop you from sharing an apple with a friend since you can either have an entire apple or none at all