Saving tomato seeds. Non-F1 varieties, obviously. Tomatoes don’t generally cross-pollinate, so it’s possible to just pick a ripe fruit and keep the seeds. Given that lots of tomato seeds have become quite expensive and that I might grow as many as sixty tomato plants each year this makes good sense for me.
In fact, the fruit doesn’t even need to be that ripe. My understanding is that once the fruit begins to change colour, the seeds are viable. At this point (sometimes called the breaker stage) the plant significantly reduces the nutrients sent to the fruit, which also means they can be picked and allowed to ripen off the plant. The fruit contains everything required to fully ripen once it reaches this stage.
It’s also why “vine-ripened” tomatoes are actually no better than those properly ripened off the plant, though they should be better than fruit picked before they are mature and artificially “ripened” by exposing them to ethylene, which might well be the case with many industrially-produced tomatoes.
Anyhow, back to the plot… I slice the tomatoes in half horizontally and squidge/scrape the seeds out of the cavities into jars which I then half-fill with water. The lid goes on and I leave them to fester somewhere warm such as an airing cupboard for three days.
At that point there will likely be mould. They will also stink quite horribly. The water and any floating (non-viable) seeds can be poured off, the rest rinsed and dried on kitchen roll and then saved somewhere cool and dry.
Towards the end of the year I do a little germination test with mine just to check they’re good whilst there’s still time to buy another batch.