Even a mosquito doesn't get a slap on the back until it starts to work.

How to build a Mosquito trap (copied in full from http://www.diyhappy.com/quick-and-dirty-mosquito-trap/).

Materials Needed:

2000ml (2 liter) bottle
50 gram (brown?) sugar
1 gram yeast
Thermometer
Measure cup
Knife
Black paper

1. Cut the top of the bottle as shown
2. Put 200ml hot water in the bottle, stir with 50gram brown sugar. Put the sugar water in cold water to cool it down til 40C (temperature). They use a bigger container with cold water - put the small cup that they use to make sugar water in that container and that stick is a thermometer because they want the sugar water to cool down to 40C (temperature).
3. After cooling down, put the sugar water in the bottle then add the yeast.
No need to mix the yeast with the sugar water. When yeast ferments, it creates carbon dioxide.
4. When you cut the bottle, dont throw the top part away because that’d be needed for step 4 - you see they put the top upside down to fit into the bottle.
5. Put black paper around the bottle since mosquitos like dark places and carbon dioxide. This mosquito trap will then start working.

TIPS: Put the trap in some dark and humid place for 2 weeks, you’ll see the effect. You’ll have to replace the sugar water + yeast solution every 2 weeks.