Do you need more humidity in a room? I built this humidifier to help keep my mushrooms happy, but this design would work for any of your humidity needs!

Supplies

small watertight box (ex: baby wipes container)
bottle (with a diameter taking up less than half the top surface of your box
evaporating surface (preferably a humidifier filter, but a sponge also works)
electrical plug
small computer fan (with a diameter taking up less than half the top surface of your box)
electrical tape
9-volt battery
battery to wire snaps

Step 1: Assemble Materials

Assemble the materials: fan, soda bottle, sponge, and watertight box with a lid

Step 2: Marking Up the Box's Lid

Trace out the cross-section of the soda bottle and the circular cross-section of the fan area that has air-flow

Step 3: Cut It Out

Cut out both cross-sectional areas with a jigsaw

Step 4: Electrical Work

Wire the computer fan to a plug

Step 5: Prepare the Bottle

Cut slots into the neck of the bottle, to allow water to escape when it is inverted.

Step 6: Check the Fit

Do a dry-assembly to check the fit

Step 7: Assemble the Elements

Place the sponge in the box, under the fan-hole
Fill the soda bottle with water
Place the soda bottle in its hole
Invert the lid and soda bottle over the box, allowing water to flow until it reaches equilibrium
Place the computer fan over its hole
Secure the Computer fan with screws

Step 8: Extra Advice

This is a very simplistic humidifier, and this instructable is ripe for modification.

The main issues with my humidifier were:

It very inefficiently gobbled energy for an un-regulable amount of time. In future versions, I highly recommend adding a timed switch-off feature and some regulator of electricity consumption.
Sponges make extremely mediocre evaporating surfaces. On one hand, they do facilitate evaporation. On the other hand, they are a breeding ground for bacteria and never dry out.