Screencast 6.1
www.youtube.com/watch?v=tgKC-awk6p4
This screencast walks you through the concept of close packing.

Screencast 6.2
www.youtube.com/watch?v=8T-cmI7QYNg
This screencast walks you through how to calculate the packing efficiency of a cubic close-packed structure.

Screencast 6.3
www.youtube.com/watch?v=4AW9PFWaMcQ
This screencast walks you through how to identify the interstitial sites in the cubic close-packed structure.

Screencast 6.4
www.youtube.com/watch?v=BYty8cxRInA
This screencast walks you through how to calculate the limiting radius ratio.