Compiling multiple C files with gcc

I have two files, main.o and modules.o, and I'm trying to compile them so that main.o can call functions in modules.o. I was explicitly told not to try #include module.o. I really don't know what I should be doing instead. I tried a few different versions of gcc (such as gcc -x c driver main.o modules.o), but nothing I get works: the compiler continuously returns

error: called object is not a function

The .o files are my source code files (I was instructed to put my source code in files with extension .o.) What do I do to compile this?