A very similar problem is to find the slope (how steep it is) at any point on a curve. The slope of a "straight" line is easy to work out — it is simply how much it goes up or down ("y" or vertical) divided by how much it goes across ("x" or horizontal). On a "curve", however, the slope is a variable (has different values at different points) because the line bends. But if the curve was to be cut into very, very small pieces, the curve at the point would look almost like a very short straight line. So to work out its slope, a straight line can be drawn through the point with the same slope as the curve at that point. If this is done exactly right, the straight line will have the same slope as the curve, and is called a tangent. But there is no way to know (without complex mathematics) whether the tangent is exactly right, and our eyes are not accurate enough to be certain whether it is exact or simply very close.