Problem: What is the missing first step of the following process: - Over long periods of time rain can hit the limestone - And cause a acidic reaction - The limestone will slowly disintegrate - A large gap with be where the limestone was - Result in the cave.

A: Some of the rock in the ground is made of limestone

Problem: What is the missing first step of the following process: - The water freezes into ice - The cracks expand - Over time the rocks split - The rocks are now in smaller pieces.

A: Water gets into cracks in rocks

Problem: What is the missing first step of the following process: - The can is taken to a recycling center - Melted down along with other cans - The melted metal is turned into bars - Becomes material for other products.

A: A can is thrown away into a recycling bin

Problem: What is the missing first step of the following process: - Aluminum flows into streams and lakes - Acid rain dissolves minerals and nutrients in soil - Washes them away so plants can't benefit from them - PH in soil and water gets affected which can kill plants and animals - Acid rain in fog at higher elevations can strip foliage off trees.

A:
Acid rain leaches aluminum from clay particles in soil