After watching this episode about pulley's on Sid The Science Kid my daughter has been obsessed with them. I've seen some pulley based science kits that were way too overpriced and non-durable for a toddler - so I decided that we could just make our own.
For less than $3 we purchased a small pulley and a short section of rope at our local hardware store (at first the employee who assisted me thought it was a little odd to be purchasing a pulley for a 3 year old - but totally got into it, and followed me around the store giving me more ideas).
We hung the pulley on our front door (using our magnetic wreath hanger), strung one end of the rope through the pulley and tied the other end onto a small basket.
We spent the next hour or so experimenting with objects of varying weights, shapes and sizes. While you may not be able to explain the exact "physics" of a pulley system to a toddler - they will be able to learn that pulley's make it easier to lift and lower items. This continues to be a favorite "toy" in our house.
For another way to make a simple pulley with items you have around the house, check out this idea here on Education.com.
A great book to read along with this activity is, How To Lift a Lion by Robert E. Wells. Although the words may be a little "big" for your toddler, the illustrations make the concepts easy to understand.
How fun would this Melissa & Doug Tow Truck be to
play with show another way a pulley can be used?