If you know the answer to that question, you're probably from the midwest. It's corn of course. Legend has it that if the corn grows to knee-high by the July 4th the farmer will have a good season.
For the first time in many years, it's not true in a lot of places around our home in Springfield. Like across the street - see above or the background. But given the advances in farming lately, the knee-high rule has been literally a joke. In years with typical spring rainfall, shoulder-high by the fourth has been a better rule.
I'll be curious to see how this year's crop finishes.