In the 'Observing the Moon and Stars' word sort activity aligned with 1-ESS1-1, students explore the visibility of the moon and stars and how they are mostly seen at night, while the sun is visible during the day. This activity encourages students to sort words such as 'full moon,' 'sunrise,' and 'constellations' into daytime and nighttime categories, allowing them to observe and understand predictable patterns in the sky that help differentiate day from night.