Mr. Patel is showing them the different animals in the zoo, and their ability to do serious harm to men. One of the things Mr. Patel does is he takes a live goat and puts it in the tiger's cage after it hadn't eaten in days. Pi and Ravi then witness the tiger devouring the goat alive.
The quote from the book that depicts this:
"The trapdoor slid open. Silence fell again, except for beating and the click-click of the goat's hooves against the floor... ordered that Mahisha had not be fed in days... 'How could you Santoch? They're children! They'll be scarred for the rest of their lives...' 'Just wait till we're alone. You're the next goat!" - page 35-39