Cut out a circular piece of cardboard that has a diameter a little larger than the jar.
Push two holes, two inches apart, in the cardboard using a pencil. Cut off the erasers and metal sleeves from the pencils using the knife.
Sharpen the four pencil ends using the sharpener. Carefully carve away the wood at one end of each pencil, using the knife, until about 1/2 inch of graphite is exposed. The carved ends will serve as the cell electrodes.
Carefully rest the pencils in the jar on the electrodes and fill the jar with water. The pencil tips must be above the water surface. Add the baking soda and stir with a pencil until all or most of it is dissolved.
Push the the pencil tips through the cardboard holes, positioning them so that the assembly sits on the jar with the electrodes just above the bottom of the jar. Tape the pencils together below the pencil tips so that the electrodes cannot come into contact.
Connect one alligator clip to a pencil tip and connect the other clip-end to the positive "+" battery terminal. Connect the other double-ended clip between the other pencil tip and the negative "-" battery terminal. Do not let the clips come into contact or the battery will be short-circuited.
Fill the test tube with water. Stop the end of the tube tightly using your finger, then turn it upside down into the jar, releasing your finger when it is under water. Your electrolyzer is now functioning and you should see small gas bubbles forming at both electrode surfaces.
Position the tube over the electrode that connects to the negative battery terminal or cathode. When correctly positioned, the tube captures the hydrogen gas bubbles. Secure the top of the tube against the container using tape.