Ms. DaSilva and Katia Pereira , a fellow Brazilian who sells drinks , flip-flops and other South American necessities at nearby Rio Bonito , agreed that Brazil 's trendy drink of the moment is acai -LRB- pronounced ah-sigh-EE -RRB- , a palm fruit often mixed with highly caffeinated syrup from the guarana plant into a sweet purple milkshake that offers a blueberry flavor and a powerful buzz .