I love gravity bongs. They only use alot of weed to be effective too.

This is how i made my gravity bong:

1. Take a 2 liter bottle and cut the bottom off.

2. Find a 5/8 size socket wrench head and use it for the bowl. Put screen into it and it fits perfectly into the top of a coke bottle.

3. Fill up a large sink, tub, or bucket with water.

4. Place the bottle in the water so that only the top of the bottle is above water.

5. Place the bowl in the top (it should fit real snugg) and pack the bowl with weed. Dont pack the bowl huge because you wont be able to hold it down. The hits on these things are massive.

6. Light the bowl and slowly lift the bottle out of the water. You will see smoke filling the bottle.

7. Stop when there is about an inch of the bottle submerged in the water. Take off the bowl, place mouth over top, and push the bottle down into the water and inhale at the same time.

8. Hold on!

If done right 2 bowls should get you right fucked. Have fun.