I am trying to test how air will affect a car, the car is a simple design and Solid works is telling me its not watertight... I am 100% sure it is. To test this I extruded a simple shape like the one in this picture. There are absolutely no holes, just a 3D shape and its still telling me I need to create lids. When I try to create lids on any of these surfaces it does not select them or let me click the green check mark... Does anyone know what I am doing wrong? Any answers would be appreciated.