The men were tested at the water

4Then the LORD said to Gideon, “The people are still too many. Bring them down to the water, and there I will test them for you. And this is what will happen: anyone of whom I say to you, ‘This one shall go with you,’ he shall go with you; but anyone of whom I say to you, ‘This one shall not go with you,’ he shall not go.”

5So he brought the people down to the water, and Yahweh said to Gideon, “Everyone who laps the water with his tongue, as a dog laps, you shall set apart by himself; likewise everyone who kneels on his knees to drink.”

6And the number of those lapping with their hand to their mouth was three hundred men, but all the rest of the people knelt on their knees to drink water.