fruit contains seeds & if something contains something else then that something is a source of that something else ->  fruit is a source of seeds  fruit is a source of seeds & fruit is a part of a tree ->  fruit is a part of a tree that is a source of seeds  fruit is a part of a tree that is a source of seeds & an oak is a kind of tree ->  fruit is a part of an oak that is a source of seeds  fruit is a part of an oak that is a source of seeds & if something comes from something else then that something else is a source of that something -> fruit is a part of an oak where seeds come from