directories

[Diller]

while playing with permissions the other day I changed
the permission on one of my own directories so that
I only had read/write permission. After doing this
I found that I could not "cd" into this directory.
When the permission was changed to 700 I was again 
able to "cd" into the directory. My question is 
"why does one of my own directories need execute permission
for me to "cd" into it??"

Thanks in advance for all assistance.
Tony R. Diller