Hosting and serving the extra files can be costly, and the user experience is often less than ideal. To make this process easier for you and more pleasant for users, Google Play allows you to attach two large expansion files that supplement your APK. Google Play hosts the expansion files for your application and serves them to the device at no cost to you. The expansion files are saved to the device's shared storage location (the SD card or USB-mountable partition; also known as the 'external' storage) where your app can access them. On most devices, Google Play downloads the expansion file(s) at the same time it downloads the APK, so your application has everything it needs when the user opens it for the first time. In some cases, however, your application must download the files from Google Play when your application starts.