Joy in the end
Write a dark and depressing text.
End with a few words or a sentence that gives the text joy and light.
Variation
If you want, you can do the opposite. Write an optimistic text and give it a dark ending.
The form is up to you
The text can be in any form you like. Prose, poetry, postcard, meeting protocol, checklist, chat, newspaper article or anything else.If you don't want to pick the form yourself, I suggest that you write a diary entry.
3 minutes
I suggest that you spend 3 on this writing exericse.