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You've felt cold before. Sometimes it's cold outside. But what if I told you that "cold" isn't real? There's no substance or quantity called "cold" in science. We can't measure the amount of "cold" in something. Instead it's about what's NOT there. In this week's video, learn the difference between heat and temperature, why a wind makes us feel colder, and what it's like to live as a mass of jiggling atoms. This is the physics of cold.

The worlds second-largest know tree, the president, in sequoia National Park is photographed by national geographic magazine photographer Michael "nick" Nichols.

Finally they slammed the deck, and sailors ran out to chock the wheels. The pilot spooled down the rotors, going through his post-flight checklist. The co-pilot, a swarthy, olive-skinned man, clapped the pilot on the back and said, "What took you so long?" The pilot's green eyes smiled. He checked the dial of his orange Doxa dive watch, and replied with an air of a man who'd just pulled a suburban cruiser into his driveway after a leisurely commute, "We're thirty minutes early, pal."

Fog rolling over the long range mountains and burning off as it moves over Lark Harbour, west coast of Newfoundland

How wet do you get running in the rain appose to walking in the rain.

Video of a strong tornado crossing State Highway 4, just north of Langley, Kansas, southwest of Salina, Kansas on April 14, 2012. Tim Samaras is a National Geographic Explorer and severe storms researcher.