In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Thus , it may extend the current findings on the interaction between nicotine and caffeine .
### Output Text:
Thus , it may extend the current findings on the interaction between <span class="chemical">nicotine</span> and <span class="chemical">caffeine</span> . 