They built a sandcastle together, their hands covered in sand.
Meaning: They worked together to build a castle in the sand, and their hands were completely coated with sand.
Choice of Words: Sandcastle is a castle built in the sand. Together means with each other. Covered means to be completely hidden or coated with something. So, they built a sandcastle together, their hands covered in sand means they worked together to build a castle in the sand, and their hands were completely coated with sand.
Alternative Expressions
Related Expressions