I'm terrible at short stories but they are usually the easiest way to get through the writing standards for English. So I'm going to rely on the experts.
If you want to read about the Mice Quotient then check it out here. This article talks about the try fail cycle. Don't make it too easy for your characters. There should be a few try fail cycles in your story. You can read about it here.
I recommend the writing activity she asks or you can use it for a jumping point of your own story.