That's a badly worded riddle that is too open ended to have a specific solution. Many solutions can be thought up for this, which makes it not so good. A proper riddle goes like this:
What can you put into a barrel that will make it lighter and that you can see with the naked eye?
. . . . . . . . . . . . . . the answer is obviously a hole :-)
The prize is a will make you an armatar of your choice. But it has to be summer themed. So you have to get it right before the contest is over. Or else you get a halloween one.