Snowflakes! You can catch them on your tongue   or use them to make a perfect snowball. But have you ever thought about taking pictures of them? Ken Libbrecht did. He is a physics professor from California. Although the weather in California is usually warm and sunny, Ken's hobby is taking pictures of snowflakes. He travels to cold areas like Alaska and Canada to find snowstorms and take pictures of the snowflakes. "They are often beautiful, and every snowfall brings new surprises," says Ken. Ken takes pictures of snowflakes with a high-quality digital camera. He uses a microscope   to make the tiny snowflakes larger. To get the best images, Ken built a special photo-microscope just for snowflakes. He collects flakes by letting them fall onto a piece of foam board  . The pictures must be taken outdoors, so Ken wears lots of clothes to stay warm. But his fingers often get cold, because it's difficult for him to take pictures of a tiny snowflake with gloves. Each snowflake has shapes and patterns   that make it as unique   as a person's fingerprint. The hobby of snowflake photography is also unique. Next time a snowstorm blows through your town, take a closer look at the flakes before you gather them into a snowball. See if you can notice their unique patterns. Millions of snowflakes are waiting for you! ,. Which of the following isn't used when Ken works with snowflakes?
A snowball. A digital camera. A microscope. A piece of foam board.