One simple type can be made using a straw. The straw needs to be marked with a scale. To make this easier to read, use bands of different colour.
As part of the straw will be under water, remember to use permanent ink to mark your scale. Put a small amount of clay or Blu-tack at the bottom of the straw, and place it in a cup of water. Observe how the straw floats. If it sinks, then remove some of the Blu-tack. If it is unstable then you need to increase the amount of Blu-tack or you could place a small nail inside the straw.
Once your pupils have finalised their designs, encourage them to use their hydrometer to compare different liquids, such as fresh water with salt water. By increasing the amount of salt by set amounts your students will be able to make a graph showing what happens. They should then be able to gauge the salinity of a mystery water sample that you have prepared in advance. You can experiment with other things such as sugar or washing up liquid.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uk