Puzzles
Body  Title  

hollowed out sphere  
Resistors 5  
Naughts and Crosses  
Cylindrical Fuel Tank  
liquid in a cylinder  
Liquid in a cone  
Liquid in a sphere  
This is a simple counting exercise; just make sure you don't miss any! 
Triangles  
Flag Colourings 2  
What is the volume of a regular octahedron? 
Octahedron Volume  
This is the obvious follow up to resistors2. In this question, you absolutely must use the Kirchhoff laws.

Resistors3  
The natural successor to resistors1. Some interesting strategies and shortcuts start to appear in this problem. We also need to start thinking in terms of Kirchhoffs laws, if not necessary applying them.
The first law says that at any given node $$\sum I_k = 0$$ The second law says that around any closed loop, $$\sum V_k =0$$ 
Resistors2 