Sifrei Bamidbar 87

(Bamidbar 11:5) "We remember the fish that we would eat in Egypt, free": Is it possible that the Egyptians gave them fish free? Is it not written (Shemot 5:18) "And now, go and work, and straw will not be given you": If they did not give them straw free, would they give them fish free? How, then, are we to understand "free"? "Free" of mitzvoth. R. Shimon says: The manna would change for them to any flavor they desired, except for (that of) these five things (Ibid "cucumbers, melons, leeks, onions, and garlic") An analogy: A king hands his son over to a pedagogue and charges him: See to it that he does not eat or drink anything harmful. And the son grumbles at his father, saying: It is not because he loves me, but because he does not want me to eat! The sages say: The manna changed for Israel to any thing (i.e., any flavor) they desired, but they did not see it (the desired object) with their eyes. And this is the intent of (Ibid. 6) "There is nothing. Only to the manna is our eyes." To our eyes, there is nothing — only manna in the morning, manna in the evening!