nodding vs tip of the iceberg

nodding

adj
  • Having the flower facing downwards. 

noun
  • The act by which one nods. 

tip of the iceberg

noun
  • A small indication of a larger possibility; the first part encountered of a problem that is much bigger than it seems. 

How often have the words nodding and tip of the iceberg occurred in a corpus of books? (source: Google Ngram Viewer )