When analyzing an issue mathematically, you might find it useful to have a formula of some sort that allows you to plug in some (independent) value and get out some (dependent) value. Functions in your algebra and calculus classes served that purpose quite well. Indeed, the formula for converting from temperature in degrees Fahrenheit (F) to degrees Celcius (C) is given by

This way, the boiling point of water at \(212^o\)F maps to \(100^o\)C and the freezing point of water at \(32^o\)F maps to \(0^o\)C as everyone should know. Since only two points are prescribed in this mapping then a line appears to be a good way to correlate values from Fahrenheit and Celcius at all points in between (interpolation) and beyond (extrapolation).