Instructional Video3:40
Vlogbrothers

What State of Matter is Fire

6th - 11th
This question has been bugging me for about 20 years. And, ultimately, it still bugs me...
Instructional Video3:31
Curated Video

Bulgaria, Devil's Throat Cave

12th - Higher Ed
The cave descends straight down without branching in either direction. The water from the Trigrad River falls from a height of 42 meters down the cave' "throat", making it the highest underground waterfall on the Balkan Peninsula....