For a five year old: you have to count how many lego pieces you have in your 5 boxes so you give each of your 4 friends one box and you count the last one.
After a few days, aided by their own parents, they come back to you with a piece of paper with the number of legos in the box (hopefully they didn't keep the box :D):
Alice has 120 lego pieces
Bob has 65 lego pieces
Carla has 99 lego pieces
Dany has 87 lego pieces
Ben has 77 lego pieces
You ask your parents, because you're five and you don't know how to sum all those numbers, and they tell you that the total of lego pieces you have is 448
Map: "Friend: count all the pieces in your box"
Reduce: "Sum all the counts of all friends"
MapReduce at scale :D
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For a five year old: you have to count how many lego pieces you have in your 5 boxes so you give each of your 4 friends one box and you count the last one.
After a few days, aided by their own parents, they come back to you with a piece of paper with the number of legos in the box (hopefully they didn't keep the box :D):
You ask your parents, because you're five and you don't know how to sum all those numbers, and they tell you that the total of lego pieces you have is 448
Map: "Friend: count all the pieces in your box"
Reduce: "Sum all the counts of all friends"
MapReduce at scale :D