Open a blank document, look around you and write one or two words on what you see.
Look at the words and hit enter. Using the first entry as a starting point, let your brain take over and as fast as possible, start randomly punching in words – even nonsense – until you have 50 or so words.
This should take around 10 minutes.
—and so on..
When you finish with your list, pick 5 and make one paragraph using those words. Guess what? You just started a writing prompt without doing anything other than triggering a creative section of your subconscious.