In a group, one or more members sometimes finish a task before others; as a class, some groups finish before others. The most widely-known solution written about in CL books and spoken about in CL workshops is to use “sponge activities.” In kitchens, sponges soak up extra water on counters and tables. During CL classes, sponge activities soak up extra time when individual group members or individual groups finish ahead of others.

Sponge activities can be divided into two types: generic and activity-specific. A generic sponge activity can be used with almost any CL activity. However, before we move on, we should point out that just because students say they have finished does not mean they have fully completed their task. Sometimes, the individuals and groups that finish last complete the task the best. Please remember the CL principle of maximum peer interactions. Much of CL's magic happens in group members' discussions, when they brainstorm, teach each other, debate, add perspectives, and provide feedback. If individuals or groups end their discussions without giving themselves the time to experience this magic, CL hasn’t had a chance to work fully.

When students have thoroughly finished a task, perhaps the best generic sponge activity guides them to help peers still working on the task, whether the peers they help are groupmates or members of other groups. Such peer-based assistance flows from the CL principle of positive interdependence and its extension of cooperation as a value. In other words, groupmates and classmates feel as though they sink or swim together. No one is finished until everyone in the group or class has achieved the learning objectives and can independently do the task the group has been attempting. Other generic sponge activities include working on homework, reading books (e.g., extensive reading programs), and doing puzzles (e.g., crossword or logic puzzles).

In addition to generic sponge activities, the other type of sponge activity is activity-specific. These can also be called enrichment activities, because they encourage students to go beyond the learning that they have just completed. For instance, after reading a book, students can design a bookmark to go with the book. Students can design their own sponges, or teachers can make suggestions. For lessons concerning the Sustainable Development Goals (SDGs), a great form of activity-specific sponge involves students considering how they can use their learning to have a real-world impact related to the SDGs the class is studying. For example, students could consider how to raise money for an NGO that is working on the SDG they are studying.