That happens all the time. The answer is: One frame of reference
is moving inside the other one.
Example:
-- You're sitting still, comfortable in a soft seat, reading a book.
In the frame of reference of the seat, you're not moving, and
neither is the book.
-- The soft seat is in Row-17 of the passenger jet you're flying
in, at 37,000 feet. In the frame of reference of somebody on
the ground, you're moving at 500 miles per hour.
