Pre-ejaculate is a clear, colorless, viscous fluid that is emitted from the urethra of the penis during sexual arousal. It is similar in composition to semen but has. However, an earlier ejaculation (either from sex, masturbation, etc.) may leave some leftover sperm in the urethra. This sperm can get flushed out by precum (aka, the precum would then contain sperm), and together would have a chance to cause pregnancy.