I have yet to get more sponge balls but I took a ice punch cone. You know those fake not so realistic plastic ice cream cones with a styrofoam sponge ball top that pops off into somebody's face. I cut the bottom half off so as to hide it in my hand easier and will eventually replace the top with a sponge ball. Have it in your pocket switch into play then you say see the ball and bam. Another would actually be better for friends but hide a bottle of disappearing ink in your hand. Supposedly a ball then squirt them. Another would be to tie some thin fishing line to the string of a party popper with a safety pin on the end attached to the inside of the pocket. Do a switch making them think its a ball in your hand. Tell them to look from a safe distance and move your hand forward until their is enough tension to make it pop. Even a hand buzzer as you place balls in their hand. Even try electric touch.

I doubt it. The fire at the Station that ultimately resulted in the death of 100 people was started by a sparkler.

http://en.wikipedia.org/wiki/The_Station_nightclub_fire