A "fill the gap" writing exercise

I know. I know. I know.
I haven't put a post up in ages.
The truth is I didn't feel too inspired or too motivated.
I am still not.
But as they say, I'll get back on the horse.
Here is a writing exercise I did a couple of months back with a writing group.
We had to start writing, then pass it on to another writer who had to keep the story going until time run out. It is hard to believe that 3 different people wrote this piece but yet it is the truth.
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The telescope was small, the moon high.
He gazed up, first getting to the magic of it with his bare eyes. He had been waiting for this moment all his young life.
His father next to him proud to see his son's lifelong dream finally about to come true. All the sweat and hard work saving up all that money, to this moment, a moment of a small boy's dreams, but a moment of happiness.
The boy took a step forward into the shadow of the telescope swallowing him whole, feeling smaller than ever before. Glancing slightly back at his father.
He found focusing the telescope far more difficult that he had anticipated. A fear suddenly gripped him.
"What is the moon?" the boy asked.
Unsure of how to answer the question, his father slumped down on his knees and offered a distraction
"was that a shooting star?"
The boy jumped.
"Where?"
"It's gone now. You have to pay more attention to things!"
"But what is the moon?" the boy asked again.
"What is the moon made of, you mean? Because I can tell you straight up! It's made of cheese!!"his father added with a wink. "Have you not seen Wallace and Gromit?
"Dad, it's not funny! What is the moon?"
"Son, you know the answer to this. You've studied it long and hard enough to know. What you mean is:what if the moon is not what you want it to be?"
The boy looked up to his father and nodded.
"Well, son, you know... You'll always have expectations and they might rise to what you had in mind but then again, they might not. But the one thing is that is certain is you'll never know unless you try!"

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So have you done this type of writing exercises before and did you find them fun? successful to your liking?