Once upon a time, there was a turtle. His name was Mr. Turtle. He was not unlike this (in fact, this is him. (In fact, I think he was a she...)):

The end.

PS.

No one really knows the actually difference between turtles, tortoises, and terrapins. Since it's too hard to know the difference, we can all just settle on calling them by their scientific order, Chelonia. I will always call them turtles. #science #bitches

PPS.

I blame this confusion on Pokemon,

As you can see, Squirtle is based on the word "turtle." But Blastoise is based on the word "tortoise." What about Wartortle? Is it based on turtle or tortoise? Seriously? WTF Game Freak. I trusted you with my childhood, and this is how you repay me? Pathetic.

PPPS.

We all know the correct starter Pokemon to choose is Charmander. Unless you're playing Pokemon Yellow, in which case, it is Pikachu obviously. But this doesn't avoid the turtle/tortoise confusion. Because if you picked Charmander, then Red (because everyone owned Pokemon Blue, like me.) would pick Squirtle!

IN CONCLUSION!

If you combine the words "potato" and "turtle", you get the word: "poturtle" and the following picture from google images: