My son suggests that there ought to be a seventh "simple machine" added to the usual six; which is the Spring.
The standard six are:
1. Lever.
2. Inclined Plane
3. Wedge
4. Wheel, and axle
5. Pulley
6. Screw
These are distinguished on the basic of their basic function - each does something distinct in mechanical terms.
We could perhaps add "spring" for two reasons. Firstly because a spring does something (i.e. stores energy) that the other machines don't; and secondly because a spring is (or can be) simple.
Therefore it could be argued that a spring is also a simple machine.
Makes sense to me; albeit I suppose a spring isn't actually doing anything in terms of mechanisms; rather enabling something to be done in future.
More exactly; a spring does not fit the definition of a machine, in terms of a machine being something that changes the direction or magnitude of a force.
On the other hand; by a common sense understanding of what constitutes "a machine"; the spring does seem to be an appropriate addition.
(As nearly always; answers depend on assumptions: on prior definitions and exclusions.)
BTW - Such simple machines can be used (with intelligence and planning, and some joinery ability) to accomplish the apparently impossible... Such as quickly (within minutes) moving a 439 pound box containing fragile precision machinery, from the back of a van down steps into a basement, equipped only with the strength of a single 13 year old boy...
