In addition to teaching science I also teach my homeroom reading/ELA. One of the most common complaints I hear with fourth graders, when faced with a writing assignment, is "I can't think of anything to write about!"
I always tell children, "Fair enough. Make it up then." This always blows my students away.
As a teacher I am not grading if they are telling the truth or not in their writing. I am grading their ability to develop an idea, write coherently, use proper punctuation, etc.
To drive this point home I play a writing game called Fool the Teacher. I assign a short writing prompt, i.e. write about one of your pets. I leave the room (briefly) while they decide who, at each table group, is going to lie and who is going to tell the truth in their writing. The students call me back in and I tell them to write.
I sit at a table and as they finish they bring it up to me and I read it. I have to decide if they are lying or telling the truth. If I guess wrong they get a starburst treat.
My kids LOVE this game and ask to play it all the time. The quality of their writing has improved drastically (learning how to add just enough detail to convince me but not so much as to make their writing unbelievable). When I manage to guess correctly the child wants to know what they did wrong.
Some people may not feel comfortable having students make up their writing but I'm fine with it. This was definitely one of my better writing ideas (wish I had thought of it a couple of years ago!)