The number of configurations of a polymer is reduced in the presence of an obstacle. The resulting loss of entropy adds a repulsive component to other forces generated by interaction potentials. Recent advances in single molecule manipulation techniques have brought the accuracy of position and force determination into the range where the measurement of relatively small deformations becomes possible. In these situations the detailed shape of probes to which the molecule is attached must be taken into account. I will discuss the influence of probe shapes on the elastic response. When the obstacles/probes have scale-free shapes, exact expressions for the forces (independent of polymer details) can be found