In this book, the Markov Chain algorithm simultaneously generates a poem and a walk along trees in the neighbourhood Las Letras in the centre of Madrid. Despite the impression that there are few trees in the neighbourhood, the algorithm counts 460 of them. Markov Chain was designed in 1906 by Andrey Markov, a Russian mathematician who died in 1992. This algorithm is at the basis of many softwares that generate spam. It is used for systems that describe a series of events that are interdependent. What happens depends only on the previous step. That is why Markov Chains are also called "memoryless".