When I create a 3D model in Blender or Milkshape, export it to MDL file, load it and a 3D character into WED, create a walk-through script, build the level/map, and try to run the game, the character falls through the floor of the 3D model. How can I fix the problem? What's the difference between "Build Simple Map" and "Default Build" in "Map Compiler" dialog box? I know that "Build BSP Map" takes the longest time.