ELI5 Pluto - is it a planet or not?

Why did Pluto suddenly became a dwarf planet? Why did they change it in 2006, and who defined what makes a planet and what doesn't? Why did we get so excited about new photos of Pluto a few years ago?

 

... We explain like i'm five!

 

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: silentwalrus92, calc3, ursus1337, sirkendizzle, lithium, staggerlee024, mason11987, lonewolf220 & bobbycorwin123.

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