Question 763502
Functions can only have one {{{y-value}}} per {{{x-value}}}.
 
{{{y=x^2}}} is a function; works because it has two {{{x-values}}} per {{{y}}}

{{{x=y^2}}}=> {{{y}}} will be {{{sqrt(x)}}} or {{{-sqrt(x)}}}; it's not a function because has two {{{y-values}}} per {{{x}}}, which, for functions, is {{{against}}} the rules 

This will make sense if you graph {{{y=x^2}}} and {{{x=y^2}}} side by side.
Now try the "vertical line test".


{{{ drawing(600,600, -10, 10, -10, 10,  blue(line(2,10,2,-10)),graph( 600,600, -10, 10, -10, 10,sqrt(x),-sqrt(x), x^2)) }}} 

as you can see, the vertical line intersects {{{x=y^2}}} in two points