The sun warms a lake. The water turns to vapor and rises. High up, cool air turns the vapor back into tiny droplets. What happens next?

A monarch lays an egg on a milkweed leaf. A caterpillar hatches and eats the leaves. Then it hangs upside down and forms a green chrysalis. What happens next?

The Wright brothers built their flyer in Ohio but tested it at Kitty Hawk, North Carolina. They wanted steady ocean wind to lift the wings and soft sand for landings. Why did they test at Kitty Hawk?

On May 10, 1869, crews met at Promontory Summit, Utah, and drove a golden spike. The two railroads were joined. A trip across the country that once took months could be made in about a week. What resulted?

The Erie Canal opened in 1825. It linked the Hudson River to Lake Erie, so boats could float wheat and lumber east to New York City. Shipping cost far less than hauling goods by wagon. Why did shipping cost drop?

France gave the Statue of Liberty to the United States. Workers in Paris took the copper statue apart and packed it in 214 crates. The crates crossed the ocean by ship in 1885. What happened next?

On May 18, 1980, Mount St. Helens erupted. A blast of hot gas, ash and rock raced down the mountain and knocked flat miles of forest. Years later, small plants pushed up through the ash. Why did the trees fall flat?

Beavers cut down small trees with their teeth. They pile the branches and mud across a stream until the water cannot flow past. The water spreads out behind the pile. Why does a pond form behind the dam?

Wolves returned to Yellowstone in 1995. Elk no longer stood in one spot by the streams for long, so young willows had a chance to grow tall. Beavers, which build with willow, came back too. Why did the willows grow back?

An iron nail left outside gets wet again and again. Air and water slowly change the outside of the iron into a flaky orange coating. A nail kept dry in a box stays shiny. Why does the outdoor nail turn orange?

To make bread, you stir yeast into warm water and flour. The tiny yeast feed on sugar in the flour and give off gas. The gas makes bubbles, and the dough puffs up. Why does the dough rise?

Louis Braille lost his sight as a small boy. At his school, books used big raised letters that were slow to feel. By age 15 he had finished a code of six raised dots that fingers could read quickly. Why did he make the dot code?

A honeybee lands on a flower to drink nectar. Yellow pollen sticks to the fuzz on her body. When she flies to the next flower, pollen rubs off there, and that flower can grow fruit. Why can the second flower grow fruit?

Rain seeps into a crack in the road. On a cold night the water freezes and takes up more room, pushing the crack wider. Cars roll over the weak spot and pieces break loose. Why does the crack get wider?

In the 1930s, farmers on the Great Plains had plowed up the thick prairie grass. Then years of drought dried the bare soil to powder. Strong winds lifted it into huge dust storms. Why did the wind carry off so much soil?

First, rinse the dry beans. Next, soak them overnight in cold water. Then simmer them for two hours. Finally, stir in the salt. How is this passage organized?

Rainwater used to flood the walk in front of the school every spring. The town dug a shallow ditch beside the path and filled it with gravel. Now the water drains away and the walk stays dry. How is this passage organized?

Frogs have smooth, damp skin and long legs for jumping. Toads have bumpy, dry skin and shorter legs for hopping. Both hatch from eggs laid in water. How is this passage organized?

A storm dropped eight inches of rain on the steep hillside. The soaked dirt slid down and buried the road below, so the town had to close it for a week. How is this passage organized?

In 1804, Lewis and Clark started up the Missouri River. Late in 1805 they reached the Pacific Ocean. In 1806 they turned back and traveled home to St. Louis. How is this passage organized?