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As you probably know, astronomers use the distance that light travels in a year as a standard measuring stick for calculating the largest distances in the Universe. The nearest star to Earth, Proxima Centauri, measures 4.22 light-years from Earth. The center of the Milky Way Galaxy is 26,000 light-years away. The nearest large galaxy, Andromeda, is 2.5 million light-years away.

This is one of those questions which four year old's ask all the time. Also i would like to point out that there are plenty of other videos which address this question. However many of those fail to explain it in detail. This video covers many of those small details. I have been reading a lot about it. And while i was at it, many more questions came into picture, like why is the sun yellow, why are clouds white, why are oceans blue, etc. I have tried to cover up as much as possible in this video. I'll include all useful links below.

This is a difficult number to know for certain, since we can only see a fraction of the Universe, even with our most powerful instruments. The most current estimates guess that there are 100 to 200 billion galaxies in the Universe, each of which has hundreds of billions of stars. A recent German supercomputer simulation put that number even higher: 500 billion. In other words, there could be a galaxy out there for every star in the Milky Way.

What does it mean for a galaxy to be the most distant galaxy disovered? The numbers are staggering. Here's what you need to know.

Toward the end of his EPA career, Hudnell led a nine-agency effort to address fresh-water algal blooms, he said. The Interagency Symposium on Cyanobacteria Harmful Algal Blooms printed an 885-page report, now hosted at EPA.gov. The report linked algae growth to an increase in nutrient pollution and temperatures and the decline of algae-eating organisms.

In 2007, "that's when I decided, 'The agency's not going to do anything about this,' and I decided I could do better without the agency," he said. "At the time, I decided the best thing out there was the solar-powered circulators."

Circulation technology, he believes, is the most promising idea among the "in-lake" approaches. By cycling water up from the depths, he said, many municipalities have managed to kill off algae blooms, even in heavily affected sewage ponds.

The tactic may work by confusing the algae about its depth in the water, Hudnell said, or by making them more vulnerable to algae-attacking viruses. In either case, he said, he has seen results.

A scientist takes a big risk to get a lava sample from the rim of a lava lake.

A neat ASAP video that explains why people lose their hearing over time. Also does a nice job showing the connection between frequency and pitch.

Hank does his best to convince us that chemistry is not torture, but is instead the amazing and beautiful science of stuff. Chemistry can tell us how three tiny particles - the proton, neutron and electron - come together in trillions of combinations to form ... everything. In this inaugural episode of Crash Course Chemistry, we start out with one of the biggest ideas in chemistry ever - stuff is made from atoms. More specifically, we learn about the properties of the nucleus and why they are important to defining what an atom actually is.

Huw James took a trek up Mount Etna and decided to show us what actually happens when a volcano erupts!

With a little bit of help from Dr Suze Kundu and using a simple demonstration heating a glass beaker of wax, stone, sand and water we can see what happens when a volcano erupts.

We can actually tell a lot about a volcano looking at the lava that comes out. If the lava is quite dense and thick we know it contains a lot of the compound silica. If it is less dense it has less silica and spreads out a lot more.

Thick lava will generally erupt from one vent and follow one flow down the side of the volcano. Thinner lava, lava that is less dense, generally erupts from the surrounding magma chambers and flows in many different channels.

Have you ever found yourself in the path of molten lava? Has the volcano you're climbing suddenly erupted and you don't know what to do? Well keep watching because Huw James has all you need to know.

The earth is built on tectonic plates that move around on the mantle. Sometimes these plates move around and come together to form mountain chains like the Himalayas, some rub together and set off earthquakes, and some like Mount Etna, interact and one plate goes underneath the other.

A Dropbox folder being used to store content for the science department at Salem Middle School.

The New York State Science Regents Test.

A page from the DPI website that includes released forms of the MSL tests for science

A collection of science assessment questions from the American Association for the Advancement of Science.

To understand how we hear, and how some common problems occur, it helps to understand ear anatomy. ...

'Motile' (or moving) cilia are found in the lungs, respiratory tract and middle ear. These cilia have a rhythmic waving or beating motion (see right). They work, for instance, to keep the airways clear of mucus and dirt, allowing us to breathe easily and without irritation. They also help propel sperm.

A collection of parts of the ear worksheets from the American Audiology Association.

A decent diagram of the parts of the ear that can be used with students. Includes vocabulary and a labeled and unlabeled diagram.

Found this sound quiz online. Might be worth mining for questions at some point.

A Steve Spangler video experiment that might be good for teaching students about the Law of Conservation of Energy. Involves making a solar powered oven to convert light energy to heat energy to cook S'mores.