Where do they learn this stuff

Today I was getting ready for the day, and the boys were sitting on the floor by the bathroom. Ian was talking on the phone (his play cell phone) and I asked him who is he talking to? He said it was Grandpa. I told him that Grandpa was coming to visit in a few days, and he said in the phone, see you tomorrow Grandpa. then he handed the phone to Hayden and said it is Grandpa, Hayden then started to talk to Grandpa. This whole conversation reminds me that they watch and see what we are doing and retain so much more than I even think. They continue to amaze me.