A machine is a tool used to make work easier. Simple   
machines are    simple tools used to make work easier.`
Compound machines have two or more simple machines
working together to make work easier.

In science, work is defined as a force acting on an   
object to move it  across a distance. Pushing, pulling,
and lifting are common forms of work. Furniture mover
 do work when they move boxes. Gardeners do work 
when they pull weeds. Children do work when they go
up and down on a see-saw. Machines make their work
easier. The furniture  movers use a ramp    to    slide 
boxes into a truck. The gardeners use a   hand shovel
to help break through the weeds. The children use a 
see-saw to go up and down. The ramp, the shovel,  
and the see-saw are  simple machines.

Inclined Plane
A plane is a flat surface. For example, a smooth board
is a plane. Now, if the plane is lying flat on the ground,
it isn't likely to help you dowork. However, when that 
plane is inclined, or slanted, it   can   help  you move 
objects across distances. And, that's work! A common
inclined    plane is a ramp. Lifting a heavy box onto a  
loading dock is much easier if you slide  the  box  up a
ramp--a simple machine.

Wedge
Instead of using the smooth side of the inclined plane,
you can also   use the pointed edges to do other kinds
of work. For example, you can use the edge to push   
things apart. Then, the inclined plane is a wedge. So,a
wedge is actually a kind of inclined plane. An axeblade
is a wedge.   Think of the edge of the blade.   It's the
edge of a smooth slanted surface. That's a wedge!

Screw
Now,  take   an  inclined   plane  and wrap it around a
cylinder. Its sharp   edge becomes another simple tool:
the screw.  Put a metal screw   beside a ramp and it's
kind of   hard to see the similarities,   but the screw is
actually just another kind of inclined plane. How does 
the screw help you do work?   Every turn of a    metal
screw   helps you move     a piece of metal through a
wooden space. And, that's how we build    things!

Lever
Try pulling a really stubborn weed out of the ground.  
You know, a deep, persistent weed that seems to     
have taken over your flowerbed. Using just your bare  
hands, it might be difficult or even painful. With a tool,
like a hand shovel however, you should win the battle.
Any tool  that pries something loose is a lever. A lever
is an arm that "pivots"(or turns) against a "fulcrum" (or
point).Think of the claw end of a hammer that you use
to pry nails loose. It's a lever. It's a curved arm that  
rests against a point on a surface. As you rotate the 
curved arm, it pries the nail loose from the surface.   
And that's hard work!

Wheel and Axle
The rotation of the lever against a point pries objects
loose. That rotation motion can also do other kinds of 
work. Another kind of lever, the wheel and axle, moves
objects across distances. The wheel, the   round end,
turns the axle, the cylindrical post, causing movement.
On a wagon, for example, the bucket rests on top of  
the axle. As the wheel rotates the axle, the wagon    
moves. Now, place your  pet dog  in the  bucket, and 
you can easily move him around the yard. On a truck, 
for example the cargo hold rests on  top   of    several
axles. As the wheels rotate the axles,the truck moves.