//on click, the sprite will change
    void OnMouseDown()
    {
        //if the fill toggle is set to fill cells
        if (gridManager.GetFill())
        {
            //If the cell is not marked or already filled
            if (!(marked || filled))
            {
                if (correct)
                {
                    //Fill the cell and ammend the current correct cells
                    spriteRenderer.sprite = filledSprite;
                    filled = true;
                    gridManager.AddCorrectCell();
                }
                //if the cell is incorrect
                else
                {
                    //Change the sprite and ammend current mistakes
                    spriteRenderer.sprite = wrongSprite;
                    gridManager.AddMistake();
                }
            }
        }

        //if it is set to mark (or cross) cells
        else
        {
            //Toggle if the cell is marked
            if (!filled)
            {
                if (!marked)
                {
                    spriteRenderer.sprite = markedSprite;
                    marked = true;
                }
                else
                {
                    spriteRenderer.sprite = emptySprite;
                    marked = false;
                }
            }
        }
    }