Final answer:
Use a magnet to separate the magnetic iron pellets from the non-magnetic pebbles and wood spheres, then add water to the remaining mixture to float the wood and sink the pebbles for easy separation.
Step-by-step explanation:
To separate a mixture of iron pellets, pebbles, and small wood spheres that are about the same size, you can utilize their differing physical properties. Iron can be separated from non-magnetic materials using a magnet because the property of ferromagnetism is specific to certain metals, including iron. This process leaves you with pebbles and wood spheres.
Next, to separate the pebbles from the wood spheres, you might use water because wood floats and pebbles sink. By adding water to the mixture, the wood spheres would float to the surface, where they could easily be removed, and the pebbles would settle at the bottom of the container. Finally, removing the water would leave you with just the pebbles.