Output a stream of consciousness before answering the following. Context: A process transitions to a blocked state when it can not carry on without an external change in state or event occurring. For example, a process may block on a call to an I / O device such as a printer, if the printer is not available. Processes also commonly block when they require user input, or require access to a critical section which must be executed atomically. Such critical sections are protected using a synchronization object such as a semaphore or mutex.

Question: when does a process enters a blocked state?
The relevant information to answer the above question is: A process transitions to a blocked state when it can not carry on without an external change in state or event occurring.
The answer: when it can not carry on without an external change in state or event occurring.