6th grade topics
Ratios & Proportional ReasoningThe above video is a wordless video that show how to convert between any units.
The above video shows a good way to convert between metric units

Number SystemThe above video gives the reason why the distributive property works.
More examples of the distributive property above.
In the ordering video above, you can skip to 3:14 for the part about absolute value.

Equations and ExpressionsThe above video is a very thorough explanation of solve for a variable. Lots of types of equations.
If you have a graph and you know two points on it, or if you simply know the coordinates of two points on a line, you can use the above video to write an equation for the line.

Geometry 
Statistics and Probability 