ELI5 What a cromosome is?

As father of a little girl with down syndrome I've found myself a couple of times in front of little kids asking why Penny looks like she does.

It's terrifying not to have any easy answer after so curious questions by the little guys and gals your tinny girl will have to interact.

I wonder If anyone reading this could help me find an effective short explanation or methaphore to have a kid understand what a chromosome is.

To be honest, I don't have those magic words.

I panic every time a kid asks when realizing that I can hardly find a satisfying description even for myself.

Could anybody help?

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