system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do specially shaped (hearts, smiley faces, etc) fireworks work?

Happy 4th!
gpt
so as you know when something explodes it explodes out in a sphere if you have something to serve as a resist you can shape the charge but that's not what's going on here.

so picture a ball and all along that ball's perimeter put little dots. as the ball expands (explodes) the balls along the perimeter are pushed out, each one of them have a wick and when they have been pushed a certain distance they will light up, or explode, or crackle depending on what the charge is. that's how you get a basic circle, a smiley face is just like that but inside you have a few more dots. these dots are actually fireworks in their own right, sitting inside another firework.

most the shapes you will see have some radial symmetry...this is because they are easier.

so, you might ask, how do they know how to make the wick so it goes off at just the right time? mostly...math...and some very, very, very precise packing and wick usage. fireworks can't have very large variances in the yield of the explosive it must be uniform and it must explode at the right speed. so given the speed of the explosion they set the wicks on the secondary explosive which must also be precise. variation from wick to wick will make a very weird looking smiley face...so they must be precise and accurate. 

the amount of engineering that goes into fireworks is frankly astounding.