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I forgot to mention one of my favorite things about Duplos!
The open stud on a Duplo brick is compatible with several System pieces. Most round pieces that has a long enough bottom lip can attach snugly to the inside of a Duplo stud; pieces like standard cylinders can't attach because the bottom lip isn't long enough.
In this picture, note the difference between the bottoms of the two plant pieces on the left. The one on the far left with the longer bottom section can attach to a Duplo stud, but the one next to it (with the shorter bottom section) can't attach because that bottom section is the only part narrow enough to fit inside the Duplo stud and it doesn't extend into the Duplo stud far enough to reach the area where the connection tightens.
You can also see that the plant tentacle piece fits a Duplo stud, because its bottom section is the correct width, and long enough to reach the area where the Duplo stud narrows.
