If you taste an heirloom tomato this summer you love so much that you want to grow it next year, save the seeds. It’s easy. You can simply spread the seeds on paper towels and let them dry, but you’ll get better germination if you ferment them. Here’s how. Cut a ripe tomato in half and squeeze all the seeds and juice into a plastic or glass jar. If there isn’t much juice, add a bit of water. Set the uncovered jar in a warm place out of direct light for a few days, until a thickened, whitish layer of mold forms on top. Discard this layer, tip the seeds and liquid into a sieve, and rinse well. Spread the seeds on paper towels to dry thoroughly and then seal in airtight containers, labeled with the variety and date.