It’s easy to conjure up a mental image of conjoined twins, but what do you know about them beyond how they look? There’s much to learn about these rare siblings and the extraordinary way in which they live their shared life. Indeed, these 20 eye-opening facts are a great place to start.

1. Surgery success has improved over the years, but the procedure is still rare

The mortality rate related to conjoined twins’ separation surgery depends on how their bodies intertwine. For example, those fused at the base of their spine reportedly have a 68-percent survival rate. Meanwhile, those who share a heart that’s specifically fused at the pumping chamber have never been split successfully, according to The Herald.

Surgery success rates have improved as time has gone on, but the procedures remain a very rare medical treatment. The rates of survival are pretty good, though: at least one twin will apparently make it through surgery in three out of four cases.