Grandchildren: the only people who can get more out of you than the IRS.

The joke hinges on a comparison between two forces that reliably extract resources: the tax authority (the IRS) and one’s grandchildren. Its humor comes from affectionate exaggeration—grandchildren are portrayed as irresistibly persuasive, able to elicit time, attention, gifts, and financial help even more effectively than compulsory taxation. Beneath the punchline is a familiar intergenerational theme: older adults often find themselves happily “spending” on descendants, sometimes beyond what strict budgeting would recommend. The line also lightly satirizes the inevitability of taxes by suggesting there is at least one thing even more potent than the IRS: family bonds and the desire to indulge the young.