When you open an older coconut, you need to dig in the toolbox for a hammer. On the other hand, opening a young coconut (sometimes called "green coconut") is much easier: a sharp knife and a level surface usually do the trick.
In this how-to video you can watch me take a sharp knife (and a not-so-level surface) and open a young coconut.
Well, to be truthful... I eventually get the coconut open. There's some coconut chopping hijinks in the middle there.
Some people shave the white husk away to get at the nut inside. I usually have good luck with getting a wedge in, but I think extending my arms and working on a wooden tray rather than a cutting board were maybe not my best moves.
Thus, I have to stress the need for a steady, sturdy cutting surface. It's a must when using a knife. Nobody wants to their chop hands instead of their food.
Oh... and I owe beoucoup thanks to J, my steady-handed camera guy.
Once you actually get inside the coconut, the coconut water is cool and delicious, and the soft flesh is a sweet, creamy delight when added to coconut curries, blended into Thai-style coconut soup, puréed into smoothies/frozen drinks (daiquiris, anyone?) and mixed into the pretty green chutney I made last week.