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What color is a tennis ball? Ask your classmates, and they might give you some surprising answers.

The Atlantic asked 30,000 people this question. Among the people surveyed, 52 percent said tennis balls are green, 42 percent said yellow, and 6 percent said something else.

According to the International Tennis Federation, tennis balls are yellow. So, why do most people say that they’re green?

When we first see an object in natural light, our brains recognize its true color. Even if the object is seen in different kinds of light later, our brains can still tell its true color. Scientists call this color constancy（色彩恒常性）. For example, we know that China’s flag is red. When we see it during sunset or under purple light, we still know that it is red, even if it looks like a different color.

However, the color of a tennis ball is not as pure as the flag. It appears to be a combination（结合）of yellow, a “warm” color, and green, a “cool” color.

The US-based magazine says that when our brains try to figure out what color the ball is, some people ignore（忽略）cool colors like green, blue and purple. So they see the ball as yellow. Others ignore warm colors, such as red, yellow and orange. They see the ball as green.

It is not just tennis balls that make such confusion（混淆）. In 2015, a girl posted a picture of a dress online. Some people said the dress was black and blue while others said it was gold and white. They had different opinions based on whether they ignored cool or warm colors.