The humble egg, one of nature’s perfect foods, a self-packaged, relatively inexpensive source of protein that can serve as a lowly hangover remedy or help a souffle rise to heavenly heights, is also a great teacher. It’s helping Americans understand how a free-market economy works: When demand is high and supply is low, prices rise.

As egg prices continue to climb, some grocery stores in Massachusetts are limiting how many customers can buy. The combination of bird flu and inflation might drive up egg prices by another 20% this year,