A hand needle has its eye at the blunt end because your fingers let go of it: push it through, pick it up on the other side, pull. A sewing machine can’t let go, so its needle has the eye near the point and only ever goes partway in and back out. As it starts to rise, the thread snags on the cloth and leaves a small loop hanging underneath, and a hook below catches that loop and carries it round the whole bobbin, so the lower thread ends up inside it. Then the needle lifts, the loop pulls shut, and the two threads lock together somewhere in the middle of the cloth. With every stitch, the top thread is stretched around an entire spool of the other one and drawn back up, hundreds of times a minute, without breaking. Neither thread ever goes all the way through. Each comes from its own side, and they tie the knot halfway.