I found 4 baby snails in my tank? Where did they come from!?

Jul 5, 2017
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I bought a snail a while back for my Betta fish to eat. He never ate it and so the snail lived in the tank for a while but he died a couple months ago. The other day I found the smallest baby snail floating around! I had no idea where it came from but I heard sometimes plants carry snail eggs. Maybe that? But then yesterday morning I found 3 more baby snails! Where did they come from?? Did my snail give birth before it died?? Though one of the snails is significantly younger than the other 3 (I'm assuming by size, they're all small but he's minuscule). And the snail was alone at the pet store when I got it. Anybody know where they came from?