Directory write permissions -- fail

[bof]

I've been trying to understand permissions on directories, but am having 
trouble with the "write" permission.

As I understand it, read permission (r--r--r--) on a directory allows 
the contents to be listed, write (-w--w--w-) allows files to be 
added/deleted, and execute (--x--x--x) allows access to the file contents.

To test this, I created a directory, foo, and put three files in it: 
foo1, foo2, foo3 (contents: this is foo1/2/3). I gave these files 
rwxrwxrwx permissions to prevent file permission problems.

Then I changed the foo directory permissions to r--r--r--.  I could list 
the files, but not do anything else like add/delete or "less" the file 
contents. This is as it should be.

Then I changed the directory permissions to --x--x--x. I could list the 
file contents using "less", but could not do anything else like "ls -al 
foo", or add/delete a file, as should be.

But when I changed the directory permissions to -w--w--w-,  I could not 
add a new file or delete any of the existing files, getting a 
"permission denied" message. This is not as I understand it: I should be 
able to do this.

Could anyone explain why?

Thanx.

BOF