Simple Joys

I know I write about it every year, but one of the great joys of my apartment is watching the Capitol fireworks from the balcony. They always feel personal.

Can someone explain to me how we can get delayed spanglies (the technical term, I'm sure) in fireworks? There were some waterfalls that then had red spangles. Also we got one skull, several smiley faces, letters, and, I think, a few hearts.