Although you can add plenty of new content with expansion packs and officially-released Stuff Packs, the fun doesn't stop there with "The Sims 3." Players can take it one step further by modifying the game using code and objects created by community users rather than the developers. Once you set up the game to accept mods, most will go in one central folder for use.
Available Mods for The Sims
"Modding" means different things to different people, but much is possible in "The Sims 3." For example, you can install mods that merely change the surface appearance of your Sim's wardrobe or that drastically overhaul core components of the game. More advanced mods like the latter allow you to bypass in-game restrictions, such as removing the censoring mosaic.
Installing Custom Content
Ensure "The Sims 3" is fully patched; when you open the launcher on an Internet-enabled computer, you'll receive a message if any new updates are available. When you find a mod that's compatible with your patch version, extract it with a file extractor like WinRAR or 7-Zip. Depending on what you're modding, you'll either have a .package, .sim or .sims3pack file inside; where the file goes depends on its type. In any case, you'll need to download and extract a file to allow mods in your game (see Resources). Open "Documents\Electronic Arts\The Sims 3" and extract FrameworkSetup.zip directly there; this creates the "Mods" folder where most mods will go.
Installing Specific Mods
If your mod uses a .package file, you can drop it right into the "Mods" folder. Use the "Downloads" folder in the "Sims 3" folder instead if you have a .sims3pack file; in the launcher, you can click the checkmark next to the mod you just added and click "Install" to add it to your game. Finally, .sim files will go in the "SavedSims" folder instead with no further action required. Where you find your modded content in-game depends on what you installed; for instance, custom Sims and clothes appear in Create-A-Sim, whereas new objects can be found in Buy or Build Mode.
Troubleshooting and Removing Broken Mods
If "The Sims 3" runs smoothly without mods and starts acting strangely with them, something you installed is most likely causing the problem. The best way to tell which mod is the culprit is to disable all of them and enable each of them again, one by one. The only way you can "disable" a mod would be to move it to another folder temporarily while you troubleshoot. To remove a problem mod, just delete it -- but make sure your family isn't actively using something the mod added, or else you may not be able to load the save.
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