TASK DEFINITION: Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.
PROBLEM: Sentence1: an automobile engine transforms chemical energy into mechanical energy. 
Sentence2: cars transform chemical energy into mechanical energy.

SOLUTION: chemical

PROBLEM: Sentence1: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.

SOLUTION: lightning

PROBLEM: Sentence1: Rivers are usually larger than streams. 
Sentence2: Second half becomes rocky with small stream crossings some standing water.

SOLUTION:
streams