I set it up for flossing and now I can't stand to break the chain. You open it, hit one big button to mark the day, and a number counts how many days you've strung together. Miss a day and it quietly resets, which is the annoying-in-a-good-way part. There's a five-week grid so you can see the gaps, and you pick the habit and a target yourself. Everything stays on the phone.
Tap done once a day and watch the streak flame grow before you miss one.