Plant it in a Watermelon

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I was in a Chinese restaurant, north of the Vienna, and they had turned the ladies restroom into a lovely indoor garden.  One of the trees was planted INSIDE A WATERMELON!  

How does that even work?  Was it planted when it was super tiny and then grew extra huge because of the nutrients?  It does not look like the tree was that size and then the watermelon was added to it.  And the watermelon looks so fresh, not mushy and rotting at all. How old is it?

I did some googling but I can't find anything about this.  DIY people: get on it! 

Obviously, if the tree had been a fiddle leaf fig, my mind would have been blown to bits.  

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