Domain acts as the fluid and domain but fluid just sits there

i’m trying to do some fluid simulations but the domain is acting as a fluid. for example, with a domain (cube) and a fluid (isosphere) inside the domain. when i bake, the cube turns into a fluid and falls down to the floor of what was previously the cube. the isosphere does not do anything. it just sits in where it has always been. do i have a version of blender that’s got a problem or am i just doing something wrong? thanks!

Yes, that’s normal. The Domain “turns into” the fluid.
You’ll find this described in the Blender 2.40 notes:

"Making a Simulation

You will need at least one fluid or inflow object, as well as a domain object which will be the containing space of the simulation. You can also have several obstacles and more than one fluid object. Only one domain is supported at a time. Please note that the domain should be a mesh and that only its bounding box is used. It will be replaced by the fluid simulation result at each frame."

The first time you “Bake”, you must have the Domain selected to see the ‘Bake’ button. After it is baked the first time, if you make any changes and want to re-bake, you need to have the Domain/Fluid selected.

The description about obstacles has since been updated in the 2.42 notes:

It’s possible that you have the resolution set too low to see the final result clearly. So you may need to increase the resolution slightly and/or switch the User Interface Display mode to ‘Final’ (from ‘Preview’). The ‘Geometry’ option allows you to view the original domain/limits of the Fluid Simulation.