If you go for a walk through the rocky hills of Finland’s Aland archipelago, you might come across a medium-size snake with gunmetal grey scales and darker diamonds running down its back. It looks a little bit like an adder, but it can’t be because its head is thin and tapered…

Wait, did that snake just change its head?

You can’t be sure, but now its head is definitely flat and triangular—the defining shape of adders and other vipers. That means it’s venomous. There are people living nearby, with kids and pets. You decide to kill the snake with a rock. That was a poor decision, especially since the dead snake wasn’t an adder. It was a smooth snake—completely harmless, rather