Thanks you so much for all your answers.
I do NOT claim to know Chinese. Or (to use my favorite word): I do not really know Chinese.
But I shall never forget what my Chinese-language teacher (in 1960!) told me.
To show how important tones are (in Mandarin), he gave us this sentence:
ma ma ma ma.
Each "ma" has a different tone.
The sentence means something like: Was Mother angry with the horse?
P.S. Because I cannot remember the tones, whenever I try to speak Mandarin, the listeners always ask: What?!