Ball bearings and marbles gain momentum as they follow tracks to a lower elevation. The tracks may run in a straight line or spiral around a center point. Any style will work, as long as the projectile gains sufficient force to trigger an action at the end of the track.
As the action of the machine releases a pendulum weight in one place, gravity swings it to another. As the weight strikes an object on the other side, the next action begins. More complicated contraptions may involve angled targets that deflect the weight to yet another target or targets before activating the next action.
As one gear slips into place and meshes with another gear, motion transfers in another direction and the machine triggers another stage in the process.
The seesaw's lever action allows a dropping weight on one side to raise the other side and release or strike another part of the machine. That part may go airborne or open a gate or some other action in the sequence.
Like seesaws, pulleys respond to an increase or decrease in weight to move. Containers or weights are raised or lowered, according to the designer's plan.
Waterwheels, chains, hooks and various food items can find their way into a Rube Goldberg project. Toys such as die-cast vehicles work well as mobile triggers. Contest rules govern items that may be used in Rube Goldberg machines in competition, but outside of contests, nearly any safe item can work.
Put the pieces together in a series of steps moving toward the goal. A die-cast metal car, released at the top of a track, rolls toward a bucket hanging from a pulley. The weight of the car in the bucket causes the bucket to descend and the basket tied on the other end of the rope to upend. A golf ball rolls out of the basket and strikes a gong. The vibration of the gong loosens a dog biscuit resting on top of the gong's frame and causes it to fall into a dog bowl, ready for the dog to eat.