If you can dream it you can make it! Great for modelling physical events and building machines.
Due to the steep learning curve I recommend introducing this in grade 4 and continuing with it from there. Next Gen science standards require modelling, and this could be a very useful way to do that in many cases.
How I Use It
I have used this program as a modelling tool for many different science units. Students can build almost anything allowing them to discover concepts such as buoyancy, density, friction, potential and kinetic energy, the list goes on. The learning curve is a bit steep on this, but once the students get going they can do amazing things.