Around the Internet, especially in social media, we see examples of hyperlinks in a common format that has been cynically dubbed "click bait" or "link bait".

The idea is that authors publish these links with evocative titles, trying to entice a casual browser into clicking the link and viewing the content (where the resultant loading of the advertisements presumably earn some small amount of money for the content provider).

Some examples of click bait (with links removed, to protect everyone's sanity) are:

• Watch out, these 15 baby penguins are waddling off to steal your heart.
• 31 creative life hacks every girl should know
• 20 Powerful D-Day Photos On The 70th Anniversary Of The Invasion
• 11 Sexy Star Wars Tattoos

Many of these kind of links use provocative phrasing, like "you won't believe" and "will shock you". I can see intuitively why this works to entice people to click the link.

What is the reason for giving a number, like in the examples? Is there any psychology involved in this trend?

This question comes from someone who doesn't really know anything about psychology or cognitive science; I'm just interested.