First of all, if you don't know what these are -- these are Spanish limes. Although, I'd say they aren't as sour as limes or lemons. They are a bit sweeter, with a bit of tang. They go by many names, and you can usually find these in the tropics like the Caribbean, South America, and Central America.
They fairly small in size -- a bit larger than a grape. The middle of these fruits have a large, hard seed. So, the right way to eat them is to suck them. If you want to keep all of your teeth, do.not.bite.into.these.
Here's what they look like inside:
