Brett Steenbarger does some research to see if opening gaps tend to fill during the day.  Definitely of interest to Dummy traders.

Do opening gaps tend to fill?

Weak first bar, bars 2 and 3 slide closer to the MA5 and then the bottom falls out.  Volume was very low to start the day so it probably wouldn’t have shown up in many scans.

Two different hammers at the MA5 to go long against.  Both would have worked.

Retraced to MA5, broke through resistance at $21 and never looked back.