What do you call it when someone starts singing a song, sings the first line then the next person starts...?

A round or canon would be what you're looking for. (I think) I hope I helped!! =D

Singing In A Round

It's calling singing in a round.

Like Row Row Row Your Boat? Round

Singing in "round"

Yeah, singing in rounds

A ROUND! Everyone else is right. A duet is when two people sing together.

It is called singing in Cannon

fifty one. provide lollipops to strangers. fifty two. stick slices of bread on human beings's motor vehicle antennas. fifty 3. flow to the flicks with acquaintances on your pajamas and slippers, deliver pillows. fifty 4. call your loved ones/ coworkers and tell hem you're Batman and then carry p.c. up. fifty 5. strap a vegetable on your head like a cucumber or a squash and act such as you don't comprehend it is there. fifty six. draw a eco-friendly mustache on your face and supply all and sundry who notices a Tootsie Roll. fifty seven. carry a can of whipped cream and ask human beings "would you like whipped cream on that?" fifty 8. flow back to the food market 12 cases basically to purchase one apple. for extra exciting, positioned on a diverse hat each and each time. fifty 9. positioned on a finger puppet and introduce it as your spouse/ husband. Make out with it generally. 60. positioned a candelabra on the hood of your motor vehicle in a motor vehicle park and stand in front of it reciting Shakespeare. sixty one. gel or spray your hair right into a huge wild mess and walk around like it is incredibly windy, even indoors. sixty two. shine a flashlight under your chin and talk in a frightening voice for the time of conversations, turn it off once you;re completed speaking. sixty 3. positioned on garlic around your neck and carry a wood stake. sixty 4. strike a chord each time somebody says "confident" sixty 5. draw a face on a lemon and function a staring contest with it.

You mean like Row, Row, Row Your Boat?

That's a round.

a round