Why can lizards choose to stick their fingers to surfaces whenever, but then un-stick them afterwards?

Question: Why can lizards choose to stick their fingers to surfaces whenever, but then un-stick them afterwards?
- Keywords:
  finger,
  lizard
Asked by Olaf to Jonny, Paul on 26 Jun 2015.
- Paul Brack answered on 26 Jun 2015:
  
  Here’s an answer: http://news.sciencemag.org/2002/08/how-geckos-stick-der-waals
  
  Basically, very weak forces between molecules called van der Waals forces are enough to hold the geckos feet, or rather all of the tiny hairs on their feet, to surfaces. If they move their feet away, the forces are so weak that they come off.

Question: Why can lizards choose to stick their fingers to surfaces whenever, but then un-stick them afterwards?