Horizontal line test

When you draw a picture with a bunch of lines that cross each other, sometimes it can be hard to figure out where they start and stop. The horizontal line test is a way to help us figure out if we can tell where the lines start and stop.

Imagine you draw a picture of a roller coaster, and you want to figure out if it's possible for the roller coaster to go up a hill and then back down without ever going backwards or stopping in the middle. To use the horizontal line test, you draw a line going straight across the picture, from left to right, like a horizon. If the roller coaster crosses that line only one time, then it's okay - that means it never goes backwards or stops in the middle. But if it crosses the line more than once, that means it goes back over a part of the track it already went over, or it stops in the middle, which means it wouldn't really work as a roller coaster.

The horizontal line test is used in math to help us figure out if a picture of a bunch of lines is a graph of a function. A function is a rule that takes in some information (like a number) and gives out another piece of information (like a different number). When we draw a graph of a function, we use lines to connect the points that the rule gives us. If we use the horizontal line test and the line we draw crosses the graph only one time for every horizontal line we draw, then we know the graph is a function. But if the line crosses more than once, that means the graph isn't a function, because it breaks the rule that a function can only give out one answer for every input. The point of the horizontal line test is to help us understand graphs better and avoid mistakes.