Really, there isn't a trick to it. All falls into place if you have enough tubes. The only way to get test tubes is to watch ads. You have to let ad run to end. To close out the ad, you won't get the extra tube, or you'll have to repeat ad again, watching it to the end. But it doesn't close ad when you've reached the end. So you're sitting there watching ad to the end, but it doesn't return to the end.