This is a mangosteen. To open it, rip off the bottom half. The outside of a mangosteen is soft, so it's easy to get it open.
This is the inside of a mangosteen. The edible white part is usually divided into 5-7 segments. The biggest segment has a seed inside of it. The purple part is not edible. Even just licking it gives you cotton-mouth like crazy. The purple juice also stains everything it comes into contact with.
In order to avoid cotton-mouth, squeeze the mangosteen so that the white part partially comes out, then pick it out with your teeth. You will need to make a face similar to or exactly the same as the one I am making.