答：当两个商标相似，并且它们所使用的商品或服务相关，以至于消费者可能会误认为它们来自同一来源时，就存在混淆的可能性。

A: The USPTO determines that a likelihood of confusion exists when both (1) the marks are similar, and (2) the goods and/or services of the parties are related such that consumers would mistakenly believe they come from the same source.