Cut the end off of one of the two liter bottles.

Shove this piece into the top part of the bottle.

This makes a chamber to hold some weight (dirt or something).
(see last picture for a new way to do this.)

You will want to glue or tape it into the bottle.

The weights can be inserted thru the opening and closed with the lid.

Rockets are like arrows: they have a heavy point and feathers (fins) to slow down the back end. You don't want too much weight, though, or it won't fly far (F = ma, remember?). We have scales. 250g total weight will work (whole rocket). (As little as 220 g worked well, too. We don't yet know the minimum possible weight.)

Put the other bottle into the first, as shown
and duct tape the two bottles together.

This second bottle will be your engine so DON'T DAMAGE IT!
DO NOT get dirt, rocks, or sand into the engine bottle.
It could damage the launcher.

Cut 3 or 4 small fins and attach them with duct tape.
(These fins are perhaps just a bit too bit,
so they add too much drag.)

Here's a close up of the fin and its beautiful duct tape job.

Notice how far away from the end the fin is - that's important!

If the fin goes too far down it will interfere with the launcher and we'll have to trim them to fit.

OK - here's an innovation! This student was searching for a way to concentrate the weight in the nose of the rocket and so it wouldn't shift. This is a bolt with washers for weight. If you use a long enough bolt you can adjust the total weight. Very smart!