A bottle should never go outside without its cap. Caps have been affixed or at least covering liquid bearing bottles for thousands of years.
Cork was probably the first widely used material to keep liquid safe in bottles. Cork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from the Cork Oak which mostly grows in southwest Europe and northwest Africa. Cork is still used today, primarily for more upscale beverage protection. After the cord came the glass, and then the metal cap, sometimes screwed on, sometimes pressured to fit, and sometimes held on by various types of recloseable wiring.
  
Early metal bottle caps required an opening tool, and some soda companies helped the consumers by putting bottle openers on the outside of their vending machines. Bottle openers also became advertising pieces, as companies put the names of their products on them and gave them out for free.
All In Packaging uses hundreds of different types of caps; push pulls, flip caps, mushroom, and slanting. Most bottle caps today are plastic and attach themselves to plastic bottles. Sport caps for water bottles may be the most popular cap on the market today, offering a pull up cap that doesn’t detach from the bottle, and that be easily pushed down again to preserve the liquid inside.
For packaging it is important to choose the cap according to the features and function of the final product. You should keep in mind the product’s viscosity and choose the orifice of the cap based on that. For acidic and corrosive filling materials it is particularly important to choose appropriate materials for the cap.
All In Packaging offers flip top, disc top, push pull and softener caps, screw on or snap on. Versions made from plastic or metal are abundant. For some products it could be important to use tamper evident or child resistant caps. You should also consider the current trends, but it is more important to harmonize the color, form, style and other parameters of the cap with the plastic bottle.  Also consider the technical parameters of the cap and choose the correct neck size and neck type.
Some soda companies use caps for promotion by printing a message inside of the cap and people with the right message may win a prize. Since the bottle must be purchased to determine the message and win, people usually purchase more of the drink to increase their chances of winning. Some companies print interesting facts, riddles or games on the inside of their caps.
With the advent of the attached plastic cap to almost every bottle hastening the disappearance of metal caps collectors have begun gathering what they can find of the old stoppers and a burgeoning collectable cap market has been formed.
Because of attached plastic caps it’s getting harder and harder for a bottle to go outside without its cap on. And that’s a good thing for keeping our drinking liquids safe and covered until it’s time for us to take the next sip.