Little science experiment.

Sparks from Falling Water: Kelvin's Thunderstorm: http://youtu.be/rv4MjaF_wow

Trick Photography "Delete tourists from your travel photos"

This is an awesome trick for travel photographers.  Sometimes you’re at an amazing location, but there are people in the way of your shot.  If you want to take a picture of a landmark and people are in your shot, you will likely spend the rest of your adult life cloning people out of the shot unless you try this technique.

Step 1: Set your camera on a tripod.

Step 2: Take a picture about every 10 seconds until you have about 15 shots, depending on how fast people are walking around.

Step 3: Open all the images in Photoshop by going to File > Scripts > Statistics.   Choose “median” and select the files you took.

Step 4: Bam!  Photoshop finds what is different in the photos and simply removes it!  Since the people moved around, it fills the area where someone was standing with part of another photo where no one was there.

UPDATE: The “statistics” script mentioned here is only available in Photoshop Extended or in the Creative Cloud version of Photoshop; however, as someone mentioned in the comments, you can get a somewhat similar effect in recent versions of Photoshop Elements by going to Enhance > Photomerge > Scene Cleaner.

This tip is mentioned in an outdated article on lifehacker.

Reference: http://improvephotography.com/8806/photography-tricks/