If you're cooking a tomatoe and have extra seeds on the cutting board you can totally save them and plant them in the next available planting season!
First, put your tomatoe seeds in a tiny cup (I used a shot glass) full of water.
Sometimes giving it a carefully calculated rinse can help get out some of the tomatoe chunks.
Next, leave the seeds alone and set them aside for two days until a white film forms on the surface of the water.
This ferments the seeds enough to dissolve the "gooey" layer that encases the seed.
Carefully rinse the seeds several times and then pour out the seeds so they can dry. I poured mine into a tiny soy sauce dish. Anything will do, just make sure they can be scraped off easily once they are dry.
Set aside for a few days and then double check that they are fully dry before storing the seeds in an air right container.
I'm temporarily reusing a small sauce container until I find a sufficient glass jar.
The seeds will be ready for sewing in the next season!