Historical clue formulations

• They make sounds when they're tickled — 2025-02-16
• Piano keys — 2020-11-15
• Tickle the ___ — 2018-11-11
• Some keys, informally — 2018-08-19
• They can't stay quiet when tickled — 2017-05-20
• They may be tickled — 2010-02-28
• Many keys — 2009-06-14
• Tickled ones — 2002-11-22
• They may be tickled — 2001-05-05
• Many keys — 2000-12-15
• They're sometimes tickled — 1993-12-29