You probably don’t have a mystery snail.
You see, mystery snails (in the family Vivparidae) are illegal to own in most states in the U.S. They’re not sold at pet stores in the U.S., and most places online don’t sell them. They’re called “mystery” snails because they “give birth” to live young.
Why are they illegal? Because the mystery snail is considered an invasive species from Asia. You probably only have one if you live in Asia or if you found one in the wild and brought it into your tank. If you find one in the wild, you should report the sighting, by the way.
What most people have in their tank is an apple snail (family Ampullaridae). They lay eggs, typically above the water line. Species are often differentiated by the color of the egg.
Interestingly enough, apple snails are also illegal in the U.S., but under federal law, with the exception of the Spike-Topped Applesnail (Pomacea diffusa/bridgesii). Hopefully this is what you bought.
Even though the scientific community calls this an apple snail, the aquarium trade has referred to it as a mystery snail since 1936. The aquarium trade was in the habit of calling snails a mystery snail if they couldn’t identify them. Later when they did identify the species of the snail, the aquarium trade kept calling them mystery snails, but added some marketing spin. They referred to them as THE TRUE MYSTERY SNAIL, because “they don’t eat plants”. If only that were true.
The aquarium trade continues to call them mystery snails today to steer clear of the 2006 federal laws about most species of apple snails. However, what you bought is likely the Pomacea diffusa, the Spiked-Top Applesnail.
How they go about dropping babies versus eggs isn’t the only obvious difference. The apple snail’s spire makes up like 5% of the shell’s length, and the mystery snail’s spire is close to 33%.
Sources: USGS