Do not attempt this advanced yoga pose if you have knee or hip replacements or injuries. Everyone is built differently. Some people will never be able to comfortably sit in a full lotus (padmasana) pose. That said, it's much easier for children than for adults. Unfortunately, as we age our bodies stiffen from sitting in chairs or couches too much.
Lotus is a deep hip opener with each foot on top of the opposing thigh. Sometimes you need to use your hands to gently pull or push your top leg into place. Lotus is said to be good for the joints, bones, posture, and PMS. However, I enjoy this pose during short meditation sits.
For those who are comfortable in a full lotus, try practicing fish pose with lotus legs.
What's easier for most is the half lotus, where just one foot is on top of the opposite thigh. Finally, for those for whom half lotus is not within their comfort level, they can simply sit in sukhasana or happy pose with their legs crossed normally.