Distinguish between distance and displacement

-- Distance is the length of a trip, with the twists, turns, and circle-backs
all straightened out.  The more times you run around a circular track, the
greater distance you cover.

-- Displacement is the distance and direction from the start-point to the
end-point, regardless of the route that was followed along the way.  If you
run once around a circular track, your displacement is zero, because your
start-point and end-point are in the same place.

When is this

Ithink the answer is fusion