Would you like to make a 2 liter bottle rocket that can fly into the air using water and air pressure?  This picture only shows the trail of water after the rocket blasted off. Click here for more pictures.

Cut out 3 fins like the one below from heavy cardboard of plastic.

Measure the circumference of the bottle with a tape measure or a string.

Divide your bottle circumference by 3.  Example 351 mm divided by 3 = 117 mm.  Mark your measuring string at 117 mm intervals

Use the string to mark the places where the fins will be placed (177mm apart)

Use a ruler to .draw lines for your fins.

Tape the fins to the bottle, being careful to place them on the lines that you drew.

When you have 3 fins on your bottle, all that you need is a nose cone.

Roll up the circle to form a cone that fits the bottom of your bottle, then tape it.

To make a nose cone, cut out a circle, then cut a straight line half way through the circle.

You will need a cork and a n inflator pin.  The kind used to inflate a football.   

Tape the nosecone to your bottle and you have a rocket that is ready to launch

I will bring my pump and launch pad to my presentations, you don't have to do any of these things.
The following is for your information

I recommend that you put an extension tube on the pump to keep from shooting yourself in the head.

Clip the pump head on to the inflator  pin. 

You will need a launch pad that will hold your rocket upright.  I used coat hanger wires and a board above

Put about a half of liter of water into your bottle rocket, push the cork in as tightly as you can.  Pump air into the bottle until it blasts off.