Product review (5 stars): I freaking love this thing.
My sister had one of those spinning wheel type of dishwasher alerts fastened to her dishwasher door---the kind that you'd rotate so the word "CLEAN" would be on top when the dishes were clean, and likewise for the word "DIRTY". The problem with spinner alerts is that they are too easy to spin. If you bump into it or if a kid gets interested in it and decides to play with it, it's easy to forget whether the dishes are clean or dirty. So I knew I wanted an alert that had a sliding mechanism.

There are a couple of dishwasher alerts with sliding mechanisms available on Amazon, but I thought this was the best. It is made of plastic, so it will be more durable than a cardboard one. It also is unlikely to be switched accidentally, for two reasons: 1.) It requires a little effort to slide the alert back and forth between "CLEAN" and "DIRTY". This is a good thing. 2.) It fits neatly on the lip (edge) of the dishwasher door, nestled in the little space between the dishwasher and the counter ledge when the dishwasher is closed (so you don't see it unless you lean really close or pop the door open a bit).