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Fill the watering can with water via the water pump. Then pour this water into one of the holes and see what happens in the greenhouse.
If you pour water into the right tube, a color will come up between the plants.
There are numbers on the tubes. First find out which tubes bring up a color in the greenhouse. You can find out the order by the colors that come up in the greenhouse. Red = number 1, blue = number 2, green = number 3 and yellow = number 4
The code is 6274.