Though every story has a beginning, middle and end, the Happening story is the most simple and basic, and it is often found in picture books. There is not truly a plot, as the main character is not striving for anything, but rather reacting.
A trip downtown, to Grandma's house, to the beach can all be Happening stories. The main character sets off to the destination, reacts to all the new things he/she sees and returns home. There is almost always another character with the main character--a friend, parent, teacher. This allows for dialogue and sharing of reactions.
Sarah might go to the grocery store with Daddy, pick out some apples, stop at the post office to buy stamps, see a neighbor who is also out shopping and then return home. The Happening story is short, anywhere from 300 to a maximum of 1500 words, but more usually no more than 600.
The Happening story is an adventure to the main character, it is something he/she is doing for the first time. There should be a number of incidences in the story and the pace should be quick. Sights, sounds and smells should be incorporated into the story.
People think picture books are easy to write because they are short, but many consider them the most difficult of all books to write.