Story writing line-by-line is a much loved (and silly) activity in my home. It all begins with a simple sentence, such as "There was a chicken standing on the front porch." Each child and adult adds a new sentence after the line to create a story. It may start off like:
"There was a chicken standing on front porch." -Mom
"Mom went back into the house to get the frying pan." -Julian
"Claudia ran out onto the kitchen porch, grabbed the chicken, and hid it under her bed." -Claudia
Some stories may become more involved, some will be just plain silly. The whole point of the exercise, though, is to encourage children to write and to show them that writing is fun.
Other starting sentences are:
I was standing in the back yard when suddenly a bright light shined down on me from the sky.
I shivered as the wind howled through the trees.
The cat glared at the fish tank and licked its teeth.