If you look at the salt very closely, you can see that they make little crystals. Those crystals look like little squares, that's because of sodium. When your parents send their pictures away to be developed, the company uses different liquids to make the photos appear. Sodium is in some of those solutions. Sodium is a very important element in fertilizers.
Farmers use a lot of fertilizer to make sure their crops grow well.
You parents might even use fertilizers in their garden. Andrew Rader Studios, All rights reserved.