Pipes!
Grandpa Bird left some pipes and elbows for Isaac to play with. Isaac really enjoyed putting them together in different ways.
Last week, Isaac and his friend, Ryan experimented with a variety of different configurations. Once they built something that they liked, they turned on the water and watched to see where the water would come out. 
They put little plastic balls (slightly smaller than marbles) into the pipes at different points and experimented to see where they came out. It was fun to see them work together and make changes to the designs to get the desired results. 
Ryan and Isaac get along really well because they both have the same level of interest in the same things and they work together well.