A pencil has rolled under your chair. The mechanism you use to retrieve it is an example of a linkage, a system of links pivoted at joints. The linkage which is your skeleton has a very high mobility. The linkage which controls an up-and-over garage door or an anglepoise lamp has a lower one. Engineers measure the mobility of a linkage in terms of its degree of freedom. This is expressed by an equation which sums the contributions at each joint. In this workshop we study this equation, in two dimensions by means of Meccano, in three dimensions by means of Geomag.