What do you call a boxing on a slackline?

Punchline.

What do you call a queue of boxers?

A punchline.

How many didn't?

Ten. (Shame this one doesn't work too well for reddit - the funniest part of this joke is the third punchline enjoying people struggle to understand what the hell you're talking about.)

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What's the healthiest sport a pregnant women can play?

Kick boxing

What does a blonde see when she looks into a box of cheerios?

A. Donut seeds.