It’s probably young Thai coconut, Sara, not the hard dry brown coconut. Take a look at my recipe for Thai Coconut Soup and you will see directions for how to open them and where to buy them. The biggest trick is having the right tool. For me, it’s a heavy cleaver because it give me more control than a knife.
A cleaver is a big square knife. I have the Henckels International classic stainless steel meat cleaver. Look for it on Amazon.com for around $60. But you can also use a big butcher knife. I just feel safer with a cleaver because it’s shorter and if I’m going to attack a coconut in my kitchen, I want to make sure I don’t miss! Also, the cleaver makes fast work out of chopping vegetables… I feel like a chef.