The ovary of the flower becomes the fruit and the ovules (structures that contained the eggs) become the seeds.
Here are some examples of flowers and fruits that develop from them.
Tomato – the ovary of a flower develops into a juicy fruit.
Lemon – The outer layer of the ovary thickens into a rind.
Salmonberry – There are many ovaries in the flower of this plant. Each ovary develops into a juicy, yummy treat.
Strawberry flowers are similar to salmonberry flowers. Each ovary becomes the seed-like structure on the outside of the juicy, red fruit.