Beacon Lesson Plan Library

Crabby Mystery

Barbara Bloodworth


This activity is a fun way to introduce nonstandard measuring to students. Students will measure the length of a molting hard shell crab and estimate if the new shell will be longer or shorter after molting.


The student knows how to communicate measurement concepts.

The student measures length of objects and distance using nonstandard concrete materials.


-Saltwater aquarium
-Molting crab (1)
-Paper 12X 18
-String (ball)


1. Set-up saltwater aquarium with molting crab.
2. Gather materials for activity.


1. Tell students: Today, we have a crabby mystery

2. Show the crab in the aquarium.

3. Brainstorm with students about crabs.
a. Where they live (fresh/salt water).
b. How they grow (discuss molting).

4. Discuss with students how the crab's shell can be measured in non-standard ways. Some of the non-standard ways may include comparing the length of the crab's shell to student's hands, feet, and/or a sheet of paper, etc.

5. Inform students that string will be used to measure the crab's shell.

6. Instruct students to draw a picture of the aquarium. While students are drawing, remove crab from aquarium and call students up individually to measure the crab’s shell length with string. Students cut string to length of crab’s shell. (After measurements are complete, return crab to aquarium.)

7. Students tape the string in their picture of the aquarium. (Tape string in a straight line.)

8. Have students take turns observing the crab until the final stage of molting.

9. Class observes crab as it sheds its shell.

10. Approximately 10-15 minutes after molting is complete, remove crab from aquarium and repeat measuring with string. (Return crab to aquarium)

11. Students compare the length of this string with the first string.

12. Students tape string in their pictures of the aquarium. (Tape string in a straight line.)

13. Students draw picture of the crab around each string to show comparative size before and after molting. Assist students as necessary.

14. Students share information gathered from crabby mystery during Show and Tell concerning length of crab before and after molting.


The students draw a picture of a pre and post molting crab. Length is determined by measuring crab with string before and after molting. The teacher formatively assesses whether the student can communicate during show and tell whether the crab was shorter or longer after molting.


This activity could be adapted to Science and/or Language Arts Standards. You may want to read the book [A House for Hermit Crab] by Eric Carle. Simon and Schuster. June 1988. ISBN 0887080561
Return to the Beacon Lesson Plan Library.