When working with game development and 3D modeling, having the ability to export models in the OBJ file format is crucial for compatibility with various software and game engines. Source SDK, a set of tools provided by Valve for creating content for their Source engine, also offers the ability to export OBJ files. Here's a step-by-step guide on how to do it:

1. Open the Source SDK and load the map or model you want to export as an OBJ file.

2. Navigate to the 'File' menu and select 'Export' or 'Export as OBJ'. The specific location of this option may vary depending on the version of Source SDK you are using.

3. Choose a destination on your computer where you want to save the OBJ file, and give it a descriptive name.

4. Select any additional settings or options for the OBJ export, such as adjusting the scale or including materials and textures.

5. Click 'Export' or 'Save' to begin the export process. Depending on the complexity of the model, this may take some time.

6. Once the export is complete, navigate to the location where you saved the OBJ file and confirm that it has been successfully exported.

By following these steps, you can easily export OBJ files from Source SDK for use in your game development or 3D modeling projects. This allows you to integrate your Source engine creations with other software and engines that support the OBJ file format. Whether you're creating custom assets for a Source engine game or using Source SDK as part of a larger 3D modeling workflow, the ability to export OBJ files expands the possibilities for your creations. Happy exporting!