There are many concepts in science curriculum that are difficult to conduct experiments to prove. A great way to conduct experiments is to use computer simulations. In a simulated environment, time changes can be sped up or slowed down; abstract concepts can be made concrete and forces that cannot be seen because they are deep with in the earth or microscopic can be seen and relationships between variables can be formed. Teachers can also focus students' attention on learning objectives because the real-world environments are simplified. Students can also easily see causal relationships between variables.
Here are some good simulations on the web:
Learning Science: www.learning.org/index.htm
Visual Elements: www.chemsoc.org/viselements
Virtual Chemistry Lab: www.chemcollective.org/vlab/vlab.php
Physics Education Technology: www.colorado.edu/physics/phet/web-pages/index.html
Cell Biology Animations: www.johnkyrk.com/index.html
Interactive Human Body: www.bbc.co.uk/science/humanbody/body/interactives/3djigsaw_02/index.shtml/?skeleton