Waldoes activate instructions by passing over them. There are certain limitations and rules you must bear in mind.
- In each reactor, there is exactly one red waldo and one blue waldo. Each will only activate instructions of its own colour.
- If the atom or molecule that a waldo is carrying collides with another atom or molecule, the reactor will stop. However, empty waldos can safely pass through other waldos, even those holding atoms.
- If a waldo moves in such a way that it would force a molecule it is carrying out of bounds it will stop the reactor. This includes rotate instructions.
- If waldoes are led to the edge of the reactor grid, they will stop moving, but will activate the instruction under it each cycle.
- If both waldoes pass over an instruction in the same cycle, then the red waldo's instruction activates first.
The waldo in SpaceChem is most likely named after remote manipulators (akin to robotic hands and arms), used in industries to minimise human contact with dangourous substances, but to still allow working with materials. Remote manipulators were dubbed "waldoes" based on the Robert A. Heinlein short story "Waldo," in which a character creates such devices.