Why is this the answer?

The answer is bananas because this fun riddle is telling the life story of one of the world's most popular fruits. It uses the colors of the banana to describe when it is good to eat and when it is not. First, the riddle says, "A time when they're green." When bananas are picked from the tree and first arrive at the store, they are a solid green color. If you try to eat a green banana, it is very starchy, hard, and not sweet at all. It tastes like a vegetable instead of a fruit, which is why the riddle says this time would "cause me to frown." The banana is simply not ripe yet. Next, the riddle mentions, "A time when they're brown." If a banana sits on the counter for too long, it starts to get brown spots, and the whole peel eventually turns brown or even black. When the banana is brown and squishy, it is overripe. It might be great for making delicious banana bread, but if you try to peel and eat a brown, mushy one as a snack, you would probably "cause me to frown" again because it is just too soft and gooey. However, "just in between, for a very short while. They're perfect and yellow. And cause me to smile." This is the best part! The yellow color is the sweet spot. When a banana is completely yellow, it is perfectly ripe. It is soft, sweet, and easy to peel, making it a wonderful snack that makes everyone happy. This perfect moment is very short, which is why we have to eat them quickly before they turn brown! So, the riddle is a clever way to talk about the three stages of a banana's life: green and yucky, yellow and yummy, and brown and yuckier. It makes you think about colors and feelings all at once.