For the equation \(y=x^2\), there is only one possible value for \(y\) corresponding to any particular value of \(x\), namely, the square of \(x\). So \(y\) is a function of \(x\).
But for the equation \(y^2=x\), there can be two values of \(y\) for some values of \(x\). For example, for \(x=4\), \(y\) could have either the value of \(+2\) or \(-2\). Thus, for the equation \(y^2=x\), the variable \(y\) is not a function of the variable \(x\).