I have been collecting seed from tomatoes given to me. And from this years tomato grown in my garden. It is best to save seed from heirloom plants. Hybrids may not produce the same fruit that you just ate. They may revert to one of the parent plants. But if you like what is grown from that seed. You can collect and replant it for a few years. And it may stabilize into the same plant year after year then. It is possible any plant can cross pollinate with another type of tomato. This would create a new hybrid. So if you want to have only 1 type of tomato it is best to not grow a variety near each other.I forgot to drain my tomato seed after a few days and let it dry out. I left it to ferment from Aug 7 to Aug 21. So my seeds my not be good for next year. I should have waited only 3 or 4 days. But I was surprised the seed I collected from a tomato in my garden sprouted in the cup of water.