A physical piggy bank has one frustrating property: you can add cash easily, but you cannot see the total without counting its contents. PennyTap is an Android app that records each deposit so the total and progress toward a savings goal remain visible. The interface is deliberately small: choose a cash denomination or enter an amount, set a goal, and review a history of deposits. The interesting engineering work is in keeping those numbers honest. Here are three decisions from the PennyTap c...