How many nightmares have you ever had in your life?

Market Sizing Question & How to explain things to a kid

My 6-year-old woke up this morning and asked me a question: How many nightmares have you had in your whole life, Mommy? 

Sounds familiar? If not, then how about estimate number of shampoos used in the US each year or how many gas stations there are in San Francisco Bay Area, or one of these. And isn't it more exciting now you have to answer that in a way a kindergartner can follow? Okay, let's leave MECE out for a second and see what else we can do here.

"We'll find that out together, shall we? First, let's agree on a couple small things. We'll count the nightmares by night. If I wake up in the mid of the night from a nightmare and fall back asleep and this nightmare continued, it count as one. Yes?"

Nod.

"As long as I think it's a nightmare, even if it's not scary to you, it still counts. Yes?"

Nod. 

Great! Now let's chop up all my years into three blocks: 

1. When I was a kid, I was a happy kids just like you. I didn't have that many nightmares until I was a teenager. So until 12 let's say I had a nightmare only every 6 months, that makes it? Yes, Number A. 

2. As a teenager, I was scared of frogs and roller coasters, in addition to a few movies such as a scientist turned himself into a fly. So between 13 and 18, for every 20 dreams I had one nightmare. How do I calculate that? With some hustling we got to the Number B. 

3. In my adulthood I've learned how to rationalize scary scenes from ghost movies and cope with real life stress. I've been able to reduce on how often I have nightmares by 2/3. How much is that for the next 15 years? Number C.

OMG we have just one step left to do, adding them all up A+B+C. Ta-da - you got your answer!

Apparently I daydreamed about the second half of the conversation - Of course my kindergartner rambled away when I was just half way into teenager nightmare calculation, but I'm sure you could tell the steps I followed in solving a market sizing question. 
Step 1: Clarify the question and agree on assumptions.
Step 2: Break it down (ideally in a mutually exclusive and collectively exhaustive (MECE) manner).
Step 3: Solve each piece.
Step 4: Consolidate the result.

I hope you will never have any nightmare about interviews, or at least market sizing questions after reading this post. 

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