  In this paper we study geometric, algebraic, and computational aspects of
flexibility and infinitesimal flexibility of Kokotsakis meshes. A Kokotsakis
mesh is a mesh that consists of a face in the middle and a certain band of
faces attached to the middle face by its perimeter. In particular any 3x3-mesh
made of quadrangles is a Kokotsakis mesh. We express the infinitesimal
flexibility condition in terms of Ceva and Menelaus theorems. Further we study
semi-algebraic properties of the set of flexible meshes and give equations
describing it. For 3x3-meshes we obtain flexibility conditions in terms of face
angles.
