instruction:
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.
question:
Sentence1: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.
answer:
lightning


question:
Sentence1: Ice is frozen water. 
Sentence2: ice expands.
answer:
Ice


question:
Sentence1: Flatworms belong to the phylum Platyhelminthes. 
Sentence2: the phylum Platyhelminthes has bilateral symmetry.
answer:

Platyhelminthes