Atransform boundary occurs where two tectonic plates
collide into one another?
move apart from one another?
grind along each other? or
sit motionless against each other?

Where they slide over each other.

Transform boundaries are formed or occur when two plates slide past each other in a sideways motion.  They do not tear or crunch into each other (but the rock in between them may be ground up) and therefore none of the spectacular features are seen such as occur in divergent and convergent boundaries.

In a transform boundary, neither plate is added to at the boundary nor destroyed.  They are marked in some places by features like  stream beds that have been split in half and the two halves moved in opposite directions.

temp rises, and expansion

measure their densities same density, same sub