Question

Explain why the equation of a sideways parabola is not a function.

Answer 1

x=y^2 is not a function.

The inverse of a parabola has the same amount of vertical lines as a regular parabola's horizontal lines, which is infinite.

Any vertical lines in a relation causes so said relation to be just a relation.

Think of it-you're just rotating the parabola 90 degrees clockwise.

Answer 2

because it miserably fails the "vertical line test".  Check the picture below.