Vegetables from seed

yes, if you leave say a bean pod unpicked, the pod will continue to grow and the bean s inside mature. when the pod is ripe and dries off, the beans can be picked taken out of the pod and stored in a paper bag in a cool frost free place. next spring sow as the packet says.
i do this with all my beans and peas etc. it will work with sweetcorn, tomatoes, sweet peppers too. after all thats how the seed companies can get seed for us.
some varieties wont grow identical to the parent because they are of mixed parents. bit like us and our own children. [vaguely human and a bit like me and hubby lol]

hope this helps.

i understand with beans and things like tomatoes where the seed is inside but what about lettuce???

the lettuce produces a yellow flower similar to the oil seed rape. the flower dies and behind each is a seed head. leave it to swell and ripen then collect the seed. store and use in spring. the lettuce is in fact about 2-3 ft tall as the flower grows out the middle of the lettuce head.
fun to do but as lettuce seed is very cheap i tend not to do it.