This is a great prank to play on a housemate or family member. It involves modifying a shampoo bottle so that when squeezed the top will pop off and spurt a shampoo substitute all over them and the shower (we wouldn't want to waste their expensive shampoo now would we!).

This prank will aggravate them four-fold. Firstly, their shampoo bottle will appear blocked, causing them to squeeze it harder. Secondly, it will explode 'shampoo' (actually cornflour and water) all over them. Thirdly, if you carry it off well, they will think they wasted all their shampoo. Especially annoying since shampoo is so expensive! Lastly, if they don't notice it's not quite its normal consistency, they might rub it in their hair!

Tools Required:

sharp knife
glue gun
mixing bowl
funnel

Materials Required:

empty shampoo/conditioner bottle (must be opaque)
cornflour
water
food colouring (optional)

I've included a PDF of this instructable in the last step as pictures are quite useful in this instructable.

Step 1: Which Bottle to Choose?

You will need a bottle that's opaque (not see through) so that your victim cannot see the slightly different coloured goo in the bottle. My housemates always use the same shampoo so I can easily nab one of their empty bottles from the recycling to modify then switch it for their almost full one when they're out.

Once you've picked your bottle, rinse out the bottle, being careful not to wash off the dried shampoo from underneath the cap. Making the bottle too clean will give the game away!

Step 2: Modifying the Bottle

Picture 1 shows my bottle of choice. The cap is held on by a sharp ring of plastic around the lip of the bottle. The cap can be removed by sharply twisting it in either direction.

To get the top to pop off when squeezed, this ring of plastic needs to be filed down slighly. I found the best way was to carve little bits of the plastic off with my scalpel. I took a little bit off at a time, checking after each time how much pressure was needed to pop the cap off. When I got it so that it would pop off with a little pressure but stay on when lifted I called it quits.

Step 3: Blocking the Cap

For this to work, pressure needs to build up in the shampoo bottle when squeezed so that the cap is pushed off and 'shampoo' goes EVERYWHERE.

To do this when the cap is open the hole needs to be plugged. I found the best thing for this is a glue gun.

Carefully work a layer around the hole then mound it up over it. Don't plop it in the hole from the start or it'll be visible poking through the hole when your victim opens the shampoo bottle. If the glue is visible from the outside when you've finished, smear some shampoo over it so it looks like it's just dried in the hole.

Step 4: Replacing the Shampoo

For a slightly runnier than shampoo consistency which will explode better than shampoo, add 12 teaspoons of cornflour to a bowl.

Measure out about 250ml water (the volume of the bottle in this case) and add about 50ml to the cornflour, stirring it until it's a smooth paste.

Add the rest of the water to make a fairly runny white liquid. If your cornflour/water mix is too thick it won't explode out. Mine was the consistency of single cream (if you didn't want to do this mixing, cream would probably be a good substitute as it's cheaper than shampoo). If the shampoo you're replacing was slightly coloured add a tiny tiny drop of food colouring to give it the right tint.

Pour the mixture through the funnel into the bottle and whack the cap on.

Now you are ready! Switch it for the real deal and wait within earshot to hear their laughter/scream/anger.