Electron and molecular geometry may seem pretty simple at first, but can get more complicated as you start looking at all different kinds of molecules. In order to find the electron geometry of a molecule, consider each lone pair and atom as a point of a polyhedron. In order to find the molecular geometry of a molecule, consider only the atoms and ignore the lone pairs. A water molecule is a great example to look at. It has two hydrogens and two lone pairs, so it has four points total in the polyhedron, which makes it a tetrahedron. So the electron geometry is tetrahedral. If we ignore those two lone pairs, we can see that two hydrogens are in a sort of bent confirmation. So the molecular geometry is bent. So now I hope you can see how water can have both a tetrahedral and bent geometry. It's not that one type of geometry is more correct than the other, or that the geometry switches. It's just that there are two different types of geometry to describe two different ways of looking at the exact same molecule.