EepyBird, the Coke and Mentos guys who demonstrate how ordinary objects can do extraordinary things, brought their special brand of quirkiness to Portland’s Monument Square Thursday evening. Kicking off the Portland Performing Arts Festival, a four-day celebration of music, dance and theater, the EepyBird duo Steven Voltz and Fritz Grobe concocted a messy spectacle with multiple soda geysers soaking the square. And the crowd erupted in appreciation, too.