Soap Bottle - Breast Milk

I don't really know what to say about this one; it's unlikely you'll be in a position to fill this, and probably don't even know anyone who does.  And if they can, they probably need all of their milk for a small human.

So instead you'll have to go with plan B which is to fill it with some sort of boring white handwash and leave it in your bathroom for your guests to wonder about, sniff and maybe taste.

If nothing else it'll give them something to think about or giggle at!