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: Five phenotypic characteristics - - locomotor activity , righting ability , clonic seizure induction , stress - induced lethality , death without external stress - - were scored at various caffeine doses in drug - naive animals under empirically optimized , rigidly constant experimental conditions .
### Output Text:
Five phenotypic characteristics - - locomotor activity , righting ability , clonic seizure induction , stress - induced lethality , death without external stress - - were scored at various <span class="chemical">caffeine</span> doses in drug - naive animals under empirically optimized , rigidly constant experimental conditions . 