This is what my CliffsNotes AP Calculus review guide says:
“If a graph is ‘smiling’ at you, it is concave up; if it is ‘frowning’ at you, it is concave down.”
What an incredible academic subject. Calculus is great–the graphs have feelings!!
It comforts me that even when calculus is sad and I’m frowning at my concave-up graph, it still smiles at me.