A well known statement made by the Israelites concerning manna – we loathe this light bread – is found in this chapter. God’s response is pretty well known too, I imagine; fiery serpents came into the came and many people died. At the command of the LORD, Moses puts up a bronze serpent on a pole.

Numbers 21

When the Canaanite king Arad, who lived in the Negev, heard that Israel came by way of the spies; he fought against Israel, and took some of them prisoner.

2 Israel made a vow to the LORD, and said “If you wilt indeed deliver these people into my hand, then I will utterly destroy their cities.”

3 And the LORD listened to the voice of Israel, and delivered up the Canaanites; and they utterly destroyed them and their cities: and they called the name of the place Hormah.

4 They journeyed from mount Hor by the way of the Red sea, to bypass the land of Edom: and the people became impatient on the way.

5 The people spoke against God, and against Moses “Why have you brought us up out of Egypt to die in the wilderness? For there is no bread, neither is there any water; and we loathe this light bread.”

6 And the LORD sent fiery serpents among the people, and they bit the people; and many people of Israel died.

7 Therefore the people came to Moses, and said “We have sinned, for we have spoken against the LORD, and against you; pray to the LORD that he take away the serpents from us.” And Moses prayed for the people.

8 The LORD said to Moses “Make a fiery serpent, and set it on a pole: and every one that is bitten shall live when he looks at it.” 

9 Moses made a bronze serpent, and put it on a pole, and if a serpent bit any man, when he beheld the serpent of brass, he lived.

10 The children of Israel set forward and camped in Oboth.

11 And they journeyed from Oboth, and camped at Ije-abarim, in the wilderness which is before Moab, toward the east. 

12 From there they set out and made camp in the valley of Zared.

13 From there they set out and camped on the other side of the Arnon, which is in the wilderness that comes out of the coasts of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites.

14 So it is said in the book of the wars of the LORD “What he did in the Red sea, and in the brooks of Arnon,

15 And at the stream of the brooks that goes down to the dwelling of Ar, and lies on the border of Moab.”

16 And from there they went to Beer: that is the well where the LORD said to Moses “Gather the people together, and I will give them water.”

17 Then Israel sang this song: “Spring up, O well; sing to it:

18 The princes dug the well, the nobles of the people dug it, by the direction of the lawgiver, with their staffs.” And from the wilderness they went to Mattanah:

19 And from Mattanah to Nahaliel: and from Nahaliel to Bamoth:

20 And from Bamoth in the valley, that is in the country of Moab, to the top of Pisgah, which looks toward Jeshimon.

21 Israel sent messengers to Sihon king of the Amorites, saying:

22 “Let me pass through your land: we will not turn into the fields, or into the vineyards; we will not drink of the waters of the well: but we will go along by the king’s high way, until we are past your borders.”

23 But Sihon would not allow Israel to pass through his border: Sihon gathered all his people together, and went out against Israel into the wilderness: and he came to Jahaz, and fought against Israel.

24 And Israel struck him with the edge of the sword, and took possession of his land from the Arnon to the Jabbok, even to the children of Ammon: for the border of the children of Ammon was strong.

25 And Israel took all these cities: and Israel dwelt in all the cities of the Amorites, in Heshbon, and in all its villages.

26 For Heshbon was the city of Sihon the king of the Amorites, who had fought against the former king of Moab and had taken all his land out of his hand, all the way to the Arnon.

27 Therefore they say this proverb: “Come into Heshbon, let the city of Sihon be built and prepared:

28 For there is a fire gone out of Heshbon, a flame from the city of Sihon: it has consumed Ar of Moab, and the lords of the high places of the Arnon.

29 Woe to you Moab! You are undone, O people of Chemosh: he has made his sons refugees and his daughters into captivity to Sihon king of the Amorites.

30 We have shot at them; Heshbon is perished even to Dibon, and we have laid them waste all the say to Nophah, which reaches as far as Medeba.”

31 So Israel dwelt in the land of the Amorites.

32 Moses sent spies to Jaazer, and they took the villages, and drove out the Amorites that were there.

33 And they turned and went up by the way of Bashan: and Og the king of Bashan went out against them, he and all his people, to the battle at edrei.

34 The LORD said to Moses “Do not be afraid of him, for I have delivered him into your hand, and all his people, and his land; and you shalt do to him as you didst to Sihon king of the Amorites, which lived at Heshbon.”

35 So they struck him, and his sons, and all his people, until there were none of them left alive: and they possessed his land. 

See also: Numbers 21 in ESV, HCSB, NASB and NIV via Bible Gateway.