You're given a limited number of "Sparks" each week as your “allowance” because we want to ensure that there is real value behind both giving and receiving a Spark. (Imagine how chaotic the world would be if everyone has unlimited money).
Every week, all members of a community on Packback receive a new “allowance” of Sparks which appear in the Community Stats module (shown above) in the sidebar of the community as “Available Sparks”.
Any unused Sparks “expire” every week on Monday at 2AM CST, and a new allowance is granted. Sparks expiring helps to encourage students to give out their sparks weekly instead of saving them up.