A teachable moment is anytime that your baby is receptive and attentive to learning a sign. Usually it stems from mutual gaze that starts at an object, then at each other and then again at the object. This signals from baby to mother or father that an object has struck a cord and baby wants to learn more about it.
Teachable moments are excellent times to introduce new signs. If they are missed, they might still arise at a later time, but the novelty the item has created may be less strong producing a weaker desire for baby to learn the sign. In contrast, a highly motivated baby will do whatever possible to communicate about an item that has peeked their curiosity. This is why it’s always best to strike at these occasions rather than wait for a later time when motivation is bound to be lower. In fact, once objects become part of the regular background environment for you baby, they might prove to be even more difficult to teach than had they never been seen before. Don’t worry too much about this though, as your baby’s interests will be constantly changing over time and eventually they will come full circle to learn and relearn just about all of the things that surround them. What is important though, is to try to maximize your efficiency by seizing these moments. This is even more important with a younger mind than an older one. Young minds are ever distracted, moving from one thing to the next, while an older mind can focus and ignore outside distractions.
There are other times altogether when teaching signs is nearly impossible. Never try to teach a baby to sign when they are hungry, tired or frustrated. You want signing to be a fun activity between you and your baby.