Hi, ReadWriteStream inherits from WriteStream which hinerits from PositionableStream. When you create a ReadWriteStream with #on: you use the default definition in PositionableStream : PositionableStream class >> on: aCollection "Answer an instance of me, streaming over the elements of aCollection." ^self basicNew on: aCollection #on: is not overriden in the instance side of ReadWriteStream and therefore it's the definition of WriteStream that is called : WriteStream >> on: aCollection super on: aCollection. readLimit := 0. writeLimit := aCollection size. Put the readLimit at 0 make sense for a WriteStream ( with witch your not supposed to read ) but when you create a ReadWriteStream on a collection you would be able to read what is already in the collection, no? I wonder why #on: has not been overriden in the instance side of ReadWriteStream ? Maybe it's not a good idea to create a ReadWriteStream with a collection that already includes elements ?