Open the bottle, then recap it. This does two things: it makes it easier to open and it releases any pressure inside the bottle from altitude changes in shipping.
Once recapped tightly, use the pushpin to put holes in the sides of the bottle near the bottom. I usually do four, 90 degrees from each other to ensure that at least one will get them.
Hand the bottle to a thirsty victim.
Stand back and enjoy the show.
Why does the water only come out when the cap is loosened? Well, there are actually two factors involved here:
First, is air pressure. Pressure wants to equalize so the pressure inside the bottle and outside the bottle are equal. If you sqeeze a dribble bottle, the water will come out because the pressure inside the bottle increases due to squeezing. However, the air pressure all around is us at sea level (like here in Houston) is 14.7 pounds per square inch. So, there is NO WAY a trickle of water can push through the air and out on its weight alone. The only other way is for gravity to pull it out, which it can only do if there is air to enter at the top of the bottle to keep the pressure equal! So, if you twist off the cap, even a little, air can now enter the bottle at an equal rate that gravity pulls water out of the bottle through the hole(s), keeping pressure constant. But, you ask, why can’t the air get through the hole in the bottle? After all, if water can get out, air can get in.
Well, yes and no. Here’s the deal – water molecules really, really like each other. This is due to the way the atoms are arranged and is a little more complicated chemistry than I want to get into here, but if you want to know more, check it out here. In any case, the attraction that water molecules have to each other (and many surfaces) causes the surface of water to act as a thin film. So long as the hole is small enough, the surface tension of the water keeps air from being able to enter.
Q: If you squeeze the bottle and let go, you can see air bubbles going up from the holes, right? Doesn’t this contradict what you just said?
A: Nope. By squeezing the bottle, you increase the pressure inside so the water sprays out. When you let go of the bottle, the pressure inside decreases. It has to equalize and the only way for that to happen is for air to enter. Remember that air is pushing against the holes at 14.7 lbs per square inch. The water in the bottle doesn’t weight 14.7 pounds, so the air can easily enter the hole until the pressure equalizes.
Q: What about the top of the bottle? How can water pour out if surface tension prevents it from going out the holes?
A: This has to do with the area of the hole (how big the hole is). Surface tension in water isn’t that strong, so once the surface of water gets spread out too far, other forces can break the surface tension, like gravity pulling water down while air is able to push up and through the hole.