It could be due to boredom that I notice this, but it seems that only on airplanes ice cube have holes. Why is that?
They have the holes because of the machines that made them (check DavidG's answer), anyway this ice cubes with holes are better for planes for a few reasons:
- Because they are lighter (believe it or not, every gram in the aviation business counts, plus the ice is not made onboard, it is loaded prior to departure just like the food)
- They cool things faster due to larger area of contact with the liquid
- They also will allow more liquid to be filled in the already small airplane cups.
- Last thing, they do not tend to stick together in the ice bucket, so cabin crew won't need to smash it to break it down.
In the retail drinks trade (pubs, bars etc), machines that make these hollow ice cubes are often* used where there is not much room to store large quantities of ready-produced ice (such as in a much larger machine). The larger surface area to volume ratio means fresh ice cubes can be made more efficiently and quickly to meet demand - by the time one batch is used, the next is ready. The ice machine only needs a small storage capacity at a time to keep up.
*Of course, in warmer climates where ice is used in larger quantities, or in establishments where drinks depend on more ice (cocktail bar versus English pub), they will usually have larger capacity machines, but even then the hollow cube is common.
It's because the machine that creates the cubes has metal prongs that the ice 'grows' around.
Typically those are used to identify ice cubes made from drinkable water. Restaurants use them in most parts of the world and those are not limited to planes. Actually, we were at a steakhouse just yesterday and my smallest daughter asked exactly the same question. She probably was bored too.
The comments to your question are interesting because they outline several advantages of these. Risk of choking being reduced is a great one. Obviously cost saving and efficiency are important too.
The surface area is bigger when it is hollow. More the surface area means that it helps cool the drink faster.