I was wondering?? 🤔

Hi everyone, I was wondering. So when I'm able to find out what we're having, I want it all to be a surprise until the party. My question is, I wanted to do something with powder so when my SO kicks the ball or something it explodes, but how do I get the color in there without any of us knowing? I know the doctor will put the answer in an envelope, but who does the powder or what do I do? Haha this is my last pregnancy so I want to go all out and make it super fun!