You keep all of your shellfish under a roof down the street and it's your savings. Shellfish used to be used as money way back, and these aren't shellfish you're spending, they're ones you're saving up for later.

When you want to withdraw from your savings you force yourself to use chopsticks (ちょ) to pick up the shellfish. They're especially slippery, hard to use chopsticks too, that way it's really hard to take too much out of your savings.