How do liquid planets generate a magnetic field?

[ImaLooser]

So solid planets do not generate a magnetic field. Planets that are at least partially liquid do because they have differential rotation. I don't understand this, there is some basic concept that I'm missing. I don't even know what question to ask.

 

[Simon Bridge]

To get a magnetic field on the scale of a planet you need a net electric current.
The physical process is quite complicated...
http://en.wikipedia.org/wiki/Earth's_magnetic_field#Physical_origin
... look up "geodynamo" and "dynamo theory".
http://en.wikipedia.org/wiki/Geodynamo

Oversimplified - the fluid arts can carry an electric current while the solid planets are insulators right through. For a solid planet's rotation to give you a magnetic field there has to be an uneven charge distribution through the planet.