Assembled according to the requirements of the instrument, the first end of the catheter into the water, holding the outer wall of the test tube, if the device is airtight, the mouth of the catheter bubbles, and let go of the water back into the catheter, the catheter to form a section of the water column.
Therefore, the answer is: put the catheter end into the water; hold the outer wall of the test tube with your hand; bubbles coming out of the mouth of the catheter.