"When you say- they must hear" strips are two panels long, and compare what the parents (Darryl and Wanda) say to what the kids (Zoe, Hammie, and possibly Wren when she's older) are doing. The first panel ("When you say") features Darryl or Wanda asking the kids to do something, usually in a nice, polite manner. The second panel ("They must hear") features Zoe, Hammie, or in more cases both of them, doing something different, such as crying or giving excuses as to why they won't brush their teeth.
There was originally only five strips that worked in this fashion, from Monday, May 19 to Friday, May 23 in 2003. But on Monday, April 18, 2011, another week featured "When you say- they must hear" strips. That week had six strips and lasted until Saturday, April 23, 2011.