Make a Tennis Ball Mortar

Tennis ball mortars are some of the coolest things ever! Good thing this will show you how to construct your own.

Steps

Take the Pringles cans and clean them out thoroughly.
Take the can which will be your bottom tube and drill a 5mm hole about {{safesubst:#invoke:convert|convert}} from the bottom.
Measure the two other cans diameters and work out 50% of the cans area. A=pi*r^2. Take half of A and put it into the equation to find r.
Cut the holes out in both top cans to the size you have just calculated. Make them as round as possible using whatever means you have. You could drill multiple holes then push out the cutting, use tin snips to cut the hole, or use a jigsaw with a blade capable of cutting metal.
Sand the newly cut holes so they're nice and round. (The rounder the hole is, the further the ball will fly!)
Duct tape all the cans together, with the can with the firing hole on it at the bottom. Use about 3 layers of duct tape to seal the cans tightly together. If you wanted to you could also use glue to improve the seal.
Heavily reinforce the bottom metal plate of the mortar with tape.
Drill a large hole in the PVC tubing so that when it encases the mortar the firing hole can be seen. The PVC tube is for protection.

If it is cold in your area, try to heat up the tube for about 2 minutes
Drop 2.5mls of lighter fluid down the barrel, making sure some reaches the very bottom.
Quickly insert the tennis ball, and use a stick or your hand to hold it firmly in place while you shake the mortar for 30 seconds. Make sure the fluid doesn't leak out the firing hole.
Prop the mortar into a high firing angle and hold it in place with bricks.
Put the blast tube over the mortar making sure the inside hole can be seen. Wait 30 seconds for the gas to move through the barrel.
Put on hearing protection. Then bring a long lit match up to the hole.

Use a tennis ball you don't care about, the mortar tends to scorch the ball.
If the mortar fails to fire, tip the tube upside down and completely air it out before trying again.
After each shot the products of combustion remain in the tube, these will prevent combustion taking place. So to be sure the mortar is working at 100% be sure to completely air out the tube before shooting again.
Use for only a limited number of shootings depending on how much lighter fluid you use, and how well you reinforced the joints. The back-end is particularly susceptible to being pushed out, so reinforce it well.
This is highly combustible, use with caution!