Can you think of some "Laws of nature" that are not in the text books?

I was installing a room air condtioner and thought of a few “Laws of Nature” while clearing out the insects that took up residence in the window casing.

These might not written in text books but are true nonetheless.

1) “If there is a hole in it some creature will try to live in it.” Eg. The air conditioner water drain was plugged by a spider nest.
2) “Something always needs killing.” Eg. Grass growing between the cracks of the concrete, ants in the house, mosquitoes in the back yard, deer flies in the garage. mice in the barn.
3) “A weed is any plant growing where you don’t want it.” A maple tree seedling growing in my lawn is a weed. Petunias growing in the alyssum are weeds. Alyssum in the petunias are weeds.
4) “Nature abhors a vacuum and will try to fill it with insects.” Eg. If there are no insects in your house they will tirelessly try to get inside.

Can you think of others?
This is in Social so have at it.