"L'enfant mange le citron, tu bois du lait."
Both 'bois' and 'boit' sound the same because you don't pronounce the 's' or 't' at the end. The endings are determined by the subject (who/what is performing the verb - in this case drinking).
'I drink' = 'je bois' (singular 1st person ending) 'You drink'* = 'tu bois' (singular 2nd person ending) 'He drinks' = 'il boit' (singular 3rd person ending)
All of the above conjugations sound the same - you figure out which to use based on the context.
*When talking to one person (this form is also used informally)