Take your time on this one, as many times as you wish.
The first image is a model of a 1+1 dimensional spacetime, the time
axis is vertical, the x axis is horizontal, the speed of light is 1, the
orange cone is the light cone and the blue line is the world line of
any physical object. At any point on it the world line is confined to
the future (+) and past (-) of the light cone at that point, and the
point always moves forward in time.
In the second image, I've
done some "manifold surgery": I've made a cut each at t = -1 and t = +1
each of which extends from x=1 to x=3. Now I restitch them, but stitch
the bottom edge of the lower cut to the top edge of the upper cut, and
also the top edge of the lower cut is stitched to the bottom edge of the
What have I built?