Briefcase

Once there was a manWho carried his ideas in a briefcase
Another nudged him and said
‘What’s in your briefcase?’
‘Nothing’, he replied, ‘Except this:’
Once there was this man who was a walking, talking idea:
“Are you the train, or the track that it travels on?”
Asked the man
“Neither”, replied the passengers
“We are only clever enough to tell
The truths we need to know.”