Entropy can be collected from many hard-to-predict events in the computer: keyboard clicks, mouse moves, network activity, camera activity, microphone activity and others, combined with the time at which they occur. This collection of initial randomness is usually performed internally by the operating system (OS), which provides standard API to access it (e.g. reading from the
/dev/random file in Linux). In desktop system, laptop or mobile phone entropy is easy to collect, while on some limited hardware devices (such as simple microcontrollers) entropy is hard or impossible to be collected.