He only used the wrong tag. You can not control anthing with your mouse. It Looks like the Greenfoot demo version. The difference is that we have not working and in reality not existing black keys

Hm, just tried to change my password to 'Greenfoot'. Looks like it was not a good idea to use a capital letter. Well, I pressed f. :D

I just looked through your code and found the =+ instead of +=. But I was too slow to tell you. Another thing is, that the potions can not spawn at the top, but at the bottom, because you divide the width and height by 10 and multiply it by a number from 1 to 10. Either you should do it 1 to 9 or 0 to 10. I will try getting a better score tomorrow, already too late now. ;)