What's supposed to happen when using an object after FreeAndNil?

var MStr: TMemoryStream;
...
FreeAndNil(MStr);
MStr.Size:=0;

MStr := TMemoryStream.Create;
MStr.Free;
MStr.Size := 0;

MStr := TMemoryStream.Create;
OtherStr := MStr;
FreeAndNil(MStr);
OtherStr.Size := 0;