i have to make a rube goldberg machine....?

Question

i need a way to cut string. and i can't use fire or anything that is dangerous

Answer 1

Pretty much any method of severing a string, such as cutting or burning, would probably have to be considered "dangerous." Therefore, I would suggest that you use some sort of twine, and unravel it instead of cutting it. You just need to attach the twine to something that spins in the direction opposite that in which the twin is wound.

Answer 2

Well you can't use scissors either, because the string will just fold between the blades and not be cut,
I suggest you use a small (new and sharp) wire cutter, the kind electronic technicians use. Those blades are opposite each other, and if you use a string that's not too frizzy, but (ideally) a fishing nylon string, then it will cut.You probably need an actuator or a small pneumatic cylinder to move the cutter's handles at the required cutting force.