Yes, You Can Use An Empty Water Bottle To Separate Eggs
Cracking eggs into a bowl isn't a messy process normally, but it's a different story when a recipe calls for separating the egg yolks from the egg whites. The most crude way to separate the yolk is to place the whole egg in your hand and let the whites drain through your fingers, but this is a tricky and sticky method. Is there a way to do it without a specialized tool? That depends on whether you consider an empty water bottle a "specialized tool," but one should be easy to find, so just chug the remaining bit and you're good to get cracking. The water bottle method is a popular way to quickly and — more importantly — cleanly separate your eggs after cracking them open.
First break the eggs into a bowl as normal. Then, open your empty water bottle and squeeze the sides a little bit. Finally, stick the bottle into the bowl and press the opening against the yolk. Then, unsqueeze the bottle and it should suck the yolk inside. Now you can squeeze the bottle once more, deposit your yolk wherever you're working, and get on with the recipe.
How it works
There is a bit of physics behind how this works, involving the way air passes into and out of the water bottle when you press its sides. When you squeeze a bottle, you're forcing air out of it, obviously. But when you release your grip, closing off the end of the bottle as it attempts to return to its normal shape, it creates negative air pressure inside. The egg yolk gets caught in the middle of this back-and-forth.
When you place the water bottle's opening up against it, you form a seal, and the negative air pressure is strong enough to suck the yolk up into its opening. Luckily, the tension between the yolk and the white of the raw egg isn't that strong, which allows for easy separation. The yolk sac is also durable yet flexible enough to sluice up and inside the bottle, changing shape along the way and not breaking during the ordeal. And voila: for that brief moment, your empty water bottle just became a makeshift egg separator, and you didn't even have to get any on your fingers.
Keep your yolks in order
When would you want to separate your eggs? It's fairly common for recipes to call for egg whites on their own: egg white omelets are a popular choice for breakfast that are high in protein, and you specifically need egg whites for meringues. Meanwhile, the separated yolks are good choices for custards like flan or créme brûlée. You would also need extra yolks if you're making a hollandaise sauce for your veggies or eggs benedict.
Another motivation might be diet; you might separate them because yolks and whites are healthy in different ways. Egg whites are where most of the protein is contained, which is great if you're trying to build muscle. Meanwhile, the yolks contain omega-3 fatty acids (also found in fish oil) which are beneficial for several parts of your body, from your heart to your brain. So when you're working with eggs, you can split them up as needed, just leave an empty bottle in the cupboard for such an occasion.