You can check with your pediatrician or consult a book to figure out when, exactly, babies become toddlers.
Or you can simply live through it, in which case the signs that a baby has graduated into the annals of toddler-dom are unmistakable.
Here are 10 signs you know your baby has stopped being a baby and has definitely become a toddler:
Forehead bruises 1 of 10And other assorted bruises that make everyone you encounter secretly do a double take in horror and quickly program the number of Child Protective Services into their speed dial.
Keys 2 of 10Now you see them. Or, rather, you saw them. Same for the remote control and cell phone.
Not the toy ones, of course. Because they aren't nearly as fun to hide as the real ones.
Airplanes 3 of 10You thought a crying baby was the worst airplane seat mate?
Hi, I'm a toddler — the crying baby's older, louder, mobile, more deliberate future.
Bath time 4 of 10Hey, look what happens when I put my hands in the air and then slap them down into the water. Over and over and over. Fast.
Mommy sure looks funny when she's wet. And mad.
Whining 5 of 10The crying becomes less frequent but more specific. And louder.
That'd be whining. It's called whining.
Strollers 6 of 10Why sit in them comfortably when you can make the person pushing you carry you instead?
See, all that whining practice totally came in handy.
Grocery carts 7 of 10Why sit in them peacefully when you can use them as literal leverage to grab breakable items off the shelves and watch them shatter in spectacular fashion as you casually chuck them on the floor?
Sleep pattern 8 of 10Ha. Sleep patterns are for babies. Being a toddler means having the ability to keep everyone guessing.
As in, when will the morning nap become history? Take a guess. Yes, that's right.
Mine! 9 of 10See that thing over there? That thing I've always wanted but couldn't reach and you wouldn't get for me?
I can move now, and I can reach it. And now it's mine.
Poop 10 of 10Baby poop? Cute.
Toddler poop? Hey, isn't it someone else's turn to change the diaper?