Add some BLUB BLUB BLUB to the tub. Whether you're looking for an absorbing classroom project, or just a way to transport my little ponies around the bath, the SS Spongey is a great answer to both. I love this basic electronics project, because it also looks at propulsion and buoyancy, all while splashing around.

Let's make a boat!

What: Sponge Motorboat
Where: anywhere on about 71% of the Earth's surface
Concepts: electronics, propulsion, buoyancy
Time: ~ 30 minutes or more
Cost: ~ $1.25 per boat
Materials:
- Sponge
- DC Motor (1.5-3V)
- Battery (AA or AAA)
- Corks (or other floatation devices)
- Rubber band
- Thumbtack
- Electronic wire (with alligator clips work great)
- Firm plastic (bottles works great)
Tools:
- Scissors
- Hot glue gun and glue
- Wire stripper (preferred, but optional)
- Tub with water

Step 1: Build a Propellor

Jump right in with the part that's going to give it UMPH.

Cut off a slice of cork, and push it on your motor spindle. Check to see that it can spin freely.

Next, cut off some firm plastic, and twist the ends to make a propellor. This part takes some experimentation, so it's fun to keep some extra plastic so you can make different propellor blades. Attach it on with a thumb tack pushed through the middle of the plastic and on to the cork.

Step 2: Power Up

Snip your wire into two short pieces. If you have wire with alligator clips on either end, great! You can either use both clips, or if plan on making multiple, you can use just one clip per boat like I did here.

Strip the wire ends, and rubber band one end of each wire to either end of a battery. If you're having connection issues later, you can add a wad of aluminum foil to an end, or use copper tape for the connection instead or as well.

Step 3: Start Your Engines

Test your VROOM by clipping to one tab or your motor, and holding the wire to the other one. Is the propellor going the right way? Feel with your hand to see if it's pushing air. It will push water in the same direction, too. If you want to reverse the direction, simply switch which tab you attach the clip and the exposed wire, too.

When you have it how you like it, take the alligator clip off, and wrap the exposed wire around its motor tab. For extra security, you can add a drop of hot glue to keep it in place.

Now your alligator clip will act like your switch!

Step 4: Attach to Ship Deck

Make sure everything is seaworthy by securing it with hot glue. Glue your motor off the stern of the ship, and make sure it is secure. Add a little hot glue and press your battery somewhere near the bow to balance weight, making sure the wires can all reach the motor.

Step 5: Add Flotation Devices

Sponges may float, but you're carrying some hefty cargo. Hot glue some corks or other floaty elements (popsicle sticks, styrofoam) to keep everything high and dry-ish. You're ready to get chug chugging along!

Step 6: Take It for a Spin!

Whether you've got an ocean, lake, puddle, tub, tupperware, or massive amount of drool collected, it's time to test it out on the water. Place your motorboat in the water, attach the alligator clip, and watch it go!

You can play with the propellor design to make it take different paths. You can change the weight, flotation, orientation, and placement, to get all sorts of funny boat paths. See what you can make!

Have fun, splash around, and as always, keep exploring.