Every crime has a set of elements that the prosecuting attorney must prove in order to establish the defendant's guilt. One of these elements typically has to do with the defendant's mental state.
Usually, prosecutors must show that the defendant acted intentionally or knowingly. But, with strict liability crimes, the prosecution doesn't need to prove that a defendant intended to do something that's illegal. The prosecution doesn't even need to establish that the defendant was reckless or negligent. It's enough for a conviction to prove that the act was committed and the defendant committed it.
"Strict liability" is a concept mainly applicable to civil law. It's a way of holding someone accountable for behavior regardless of fault (such as in product liability cases). Because criminal punishment is usually reserved for those who act with a culpable (guilty) mental state, strict liability crimes are rare. But some acts produce outcomes that lawmakers want to criminally punish regardless of a defendant's state of mind.
Probably the most well-known example of a strict liability crime is statutory rape. Most states make it a crime to have sex with a minor, even if the defendant honestly and reasonably believed that the sexual partner was old enough to give legal consent.
Selling alcohol to a minor is another strict liability crime. In some states, a conviction is appropriate even if the seller honestly thought the buyer was 21 or older and tried to confirm as much.
Some traffic offenses are strict liability crimes. In many places, it doesn't matter whether a driver knowingly went over the speed limit—the plain act of speeding typically justifies a conviction.
In some instances, there might be a defense to strict liability charges other than "I didn't do it." But this is a tricky area of the law. If you face charges for a strict liability offense, contact a knowledgeable criminal defense lawyer to advise you on defenses that might apply in a given case.