Never eat a hard kiwi again
For such a small fruit, kiwis are exceedingly complicated, and it can be hard to know when kiwi fruit is ripe or not. That's partially because, as Harold McGee explains in On Food and Cooking: The Science and Lore of the Kitchen, "Their thin, hairy skin doesn’t change color during ripening," so unlike other fruit—like, say, a banana, which turns bright yellow when it's ripe and darkens as it starts to go bad—you can't tell if a kiwi is ripe by looking its skin. And though you should avoid kiwis that have obvious blemishes and bruises, just picking a kiwi with evenly colored, brown skin isn't going to be enough to tell you if it's ripe or not.
The best way to tell if a kiwi is ripe is to pick it up and feel it. The kiwi experts at the California Kiwifruit Commission have some tips on how to tell if a kiwi is ready to eat or not and what you're looking for from your kiwi. "Press the outside of the fruit with your thumb. If it gives to slight pressure, the kiwifruit is ripe," they explain. "If it doesn't give to pressure, it's not ready to eat," and it's really as simple as that.
If you accidentally bought a fruit that's still rock hard, there are some easy ways to ripen kiwi. Putting your kiwi on the counter instead of keeping it in the fridge is one way to jumpstart the ripening process. You can also put the kiwi in a brown paper or plastic bag, along with an apple or a banana. Apples and bananas excrete ethylene gas, which is a hormone that ripens fruit, so putting the unripe kiwi alongside the ethylene-producing fruit will help speed up the process. Some folks also recommend putting kiwi into a bowl of rice, which also helps trap ethylene.
And if you want to confirm that your kiwi is finally ready to eat, give it one last squeeze before cutting into it.