How did the tomato become a fruit?

Answer 1

It did not "become" a fruit - it is a fruit. Tomatoes are the fruits of the tomato plant. They're just not sweet types of fruit, but they are the containers for the plant's seeds, which is the definition of a fruit.

Answer 2

Tomato were always a fruit, the seeds develop within the body of the tomato. They are known as vegetables for culinary purposes only