As a parent of two young kids, I spend an unreasonable amount of time trying to prevent them from poking each other's eyes out (and hitting each other, and kicking each other, and biting each other, and...). But when it comes to coconuts, poke away, I say! That is, if your goal is to open a coconut and get out the meat.
Here are the essential steps for an easy, stress-relieving, coconut-opening time. These instructions apply for any brown, hard-shell coconut, which is the mature form. If you have a young green coconut, you'll need a big knife and the courage to engage in a much riskier procedure.
Step 1: Poke Out the Eyes
A coconut has three eyes, so the first step is to create a stable base by setting the coconut on a kitchen towel and punch out two of them with a screwdriver and a hammer. I think a Phillips-head works best, as a flat-head screwdriver has a broader head that can more easily get stuck inside the coconut.
Simply put the tip of the screwdriver against one of the eyes and carefully but confidently tap it with a hammer to punch out one eye. Then repeat for the second eye. Opening two eyes allows for one to drain out the coconut water within while the other takes in air, just as you need to do with a big plastic water jug to help it dispense.
Step 2: Drain the Coconut
Pour out the coconut water into a vessel. You may need to strain it of any debris or dirt, which you can do by draining the coconut through a fine-mesh strainer, or straining it later.
Step 3: Crack the Coconut
Set the coconut back on the kitchen towel and, using the hammer, whack it all over, rotating the coconut as you go. You will begin to form stress fractures all over the coconut and they will eventually widen to the point where you can fully break it open, either with the hammer or using the screwdriver to pry it open (or even with your hands if it the cracks are big enough). This can make a small mess, so be prepared to wipe up after.
Step 4: Pry Out the Meat
Using a spoon or paring knife, pry the meat from the shell pieces. Try not to let too much of the thick brown skin come with it, but if it does you can use a paring knife to trim it off. Depending on your use and preference, it's okay if some thinner brown skin remains attached.
That's it, you now have coconut water and fresh white coconut meat to use as desired.