Marcus du Sautoy meets a detective with a PhD in mathematics who has created a probability function that can help narrow down the area in which a serial killer is likely to live. The case of Jack the Ripper is outlined, and the probable street where he lives revealed. The function is then broken down and the different elements explained: the least effort principle and the buffer zone. Teacher notes: Use as an enrichment clip as part of a series of lessons on probability and relative frequency. This is a practical example of how probability functions and statistical models can be used to predict behaviour. Students could discuss what other behaviours might be able to be predicted in this way, and whether it is ever possible for us to act outside of these models, leading to a discussion of free will.