Testing takes the pattern of an initial informal chat about the project, such as the difficulties encountered and how they were overcome. Most groups find great satisfaction in the way they have been able to overcome problems, so they have considerable confidence in their ability to use their model. The initial questions asked of the students may be as straightforward as'Show where an oxygen molecule enters the bloodstream? or'What is the name of the major vein leading up to the right side of the heart?' As they relax more difficult tasks can be asked of the students, such as"Trace the path of a glucose molecule from where it enters the blood to where it leaves the capillary in a leg muscle'. Finally, for students who are still not particularly challenged by the regular testing, questions such as 'Does all the urea, in blood leaving the heart, go down to the kidneys? or"What happens to urea in the blood that travels to the brain? will make them do some thinking. As the teacher is working with only one group at a time, it is possible to keep probing their understanding until their limit is reached. Not all class members will reach the same point. This type of assessment allows for variation depending on the level of student understanding and all students can feel that they have progressed in their level of knowledge. Students are asked to assess their own understanding, and to write down the points that surprised them most about the structure or function of their body. This highlights the changes in their perceptions. Their original probes are also returned to them, serving to remind them of their initial conceptions and make them aware of the learning process they have been through. They derive satisfaction in fixing up their probes so they are 'correct'. If this is done in a different colour they have a permanent reminder of a worthwhile learning experience