Praying in your head

Jacob said his prayer "in his head" last night then asked me how Heavenly Father knows what he prays when he does it in his head.  I said that even if we just think the words then Heavenly Father knows what we are thinking.  He said,  "but I didn't think any words."  So I said, "what did you think about?"  And he said, "I just closed my eyes and didn't think about anything.  I didn't know you had to think words."  I guess he thought that praying in your head just meant not thinking for a minute.  He tried it again and got a kick out of it.  He thought it was a pretty funny thing to talk in his head!