A number line is a straight line marked with numbers in order, and it is one of the easiest tools for spotting whether a number is even or odd. On a number line that starts at 0 and counts up by ones, even numbers (0, 2, 4, 6, 8, 10, ...) always land in the same kind of spot every other step, while odd numbers (1, 3, 5, 7, 9, ...) sit in the gaps right between them.
Because this pattern repeats every two steps, once you know where 0, 2, and 4 land, you can predict every even number that comes after just by looking at the spacing on the line.