Wait.....was that explosion, Yuugi lighting her fart?

I... I don't think she lit it

I thought it was a burp. Maybe that's just me.

Nuclear fart?

Yuugi's comment is a bit ambiguous - it could also mean "Good tune today", referring to the fart/burp.

It was most definitely a fart. Just before this, Tewi was lighting someone else's fart and caught her hair on fire then Yuugi decided to step it up a notch.

It was most definitely a burp. First, "geppu" means "burp." Second, on the next page when she's trying to do it again she unmistakeably burps.