Hold down the spacebar and then mBot runs the program. Judge whether mBot is completely along the dark line. if it is, it moves straightly. Otherwise, it continues to judge whether mBot is on the left (right). If it is, it

turns to the right (left) and make mBot return to the black line. If mBot is away from the black line, it will draw back to the black line.

Repeat the cycle as this until you release the spacebar.