Press the spacebar twice, in succession. There is a strange "gravity" effect on all the particles.
THIS PROJECT WAS AN ACCIDENT! I think it is because sprite variables are set on sprites, not individual clones of sprites. However, it made a pretty neat effect! You can change the number of particles with the custom block (it is the only custom block there, so it is pretty obvious). If you want to use the particle effect from this project, just put it in your backpack! But please, give me credit for the effect, and if you feel like it, link to this project page.