The Itaipu Dam was built across the wide Paraná River and took many years and thousands of workers to finish. When the engineers closed the gates and let the water build up behind the dam, an enormous lake - called a reservoir - formed upstream. The whole thing is so long that if you walked from one end to the other it would take you about half an hour.
Inside the dam are huge machines called turbines. The weight of all that water pressing down spins the turbines, and the spinning makes electricity - just like a bicycle dynamo but enormously bigger. Paraguay uses some of that electricity itself and sends the rest to Brazil through thick cables. In fact, most of Paraguay's electricity comes from Itaipu.
The dam did not just change the river - it changed the whole area around it. A giant nature reserve now surrounds Itaipu Lake, sheltering tapirs, capybaras, deer, and hundreds of species of birds. Visitors can take boat tours on the lake and see wildlife from the water.