Protein

Proteins are foods which that your body can use as building blocks to make muscle or skin or bones. Some foods which have a lot of protein in them are meat and cheese and beans and fish.
More about protein molecules
To find out more about protein, check out this book from Amazon.com or from your library:
Nutrition: What's in the Food We Eat, by Dorothy Hinshaw Patent and William Munoz (1992). For kids.