A classic of harmless pranks appealing to young and old alike, the whoopee cushion, with its loud, realistic fart sounds, has just one thing missing:

Smell.

While my seven year old boy fantasized about extremely unsanitary ways to remedy this obvious failing, I couldn't keep my own mind from spinning. I resisted the temptation to experiment till now, but the allure of this contest and Think Geek was just too great for me.

I present here, the smelly whoopee cushion!

Step 1: What You Will Need

Approximately 12" of flexible tubing
Glass jar with lid
silicone glue (optional -- I didn't need it)
Small clamps (from office supply store)

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1/3 cup ammonia
1/2 box of matches

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Whoopee cushion

Step 2: Preparation

Cut the tubing to get one 2" piece and one approximately 10"

Drill two holes in the lid of your jars so that the pieces of flexible tubing fit snuggly.

Seal the tubing to the lid with silicone if necessary to get an airtight fit.

Step 3: Stink Bomb

With a strong pair of scissors, cut the heads off half the matches in a box, put them in your jar and cover them with the ammonia. Close the jar with your modified lid, and use small clamps to seal the two pieces of tubing.

Let sit for one week.

The sulphur in the match heads reacts to the ammonia and produces ammonium sulfide, a chemical with the odor of rotten eggs.

Step 4: Prepare the Prank

Insert the longer tube into the whoopee cushion and seal the rim with your fingers while you blow (or pump air) into the shorter tube. This will blow up the whoopee cushion with "fragrant" air, and you can now proceed to strategically place the cushion for maximum embarrassing effect.

DISCLAIMER: the match heads and ammonia are supposed to sit for a week... to make the contest deadline I only had time to to let it macerate for a couple days. At this point the resulting smell is still closer to ammonia than rotten eggs so the experiment is not fully successful -- yet. I'll report back with full results in a few days. If it doesn't work other concoctions can be used with the same basic set-up.

CAUTION: don't blow this air in any person or animals direction. It stings. Just thinking about what would happen if you sucked the air in rather than blow it out makes me hesitant to post this instructable.