Debt of Gratitude

Why does Hashem tell Moshe to instruct Aaron , 7:19, to perform the first 3 Makos ?
Why shouldn't Moshe do the Mitzva himself ?
Is a person obligated to be grateful to inanimate items such as water & earth ?
Dosent Aaron also have to be grateful to these natrual benefits ??
Now may be a good time for a gem of Torah study.
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