Historical clue formulations

• Dip ___ in — 2016-05-07
• Vol. 1 of a four-volume encyclopedia, maybe — 2013-12-05
• Dip ___ in — 2012-06-08
• Stick ___ in the water — 2012-04-23
• Stub ___ — 2011-05-01
• Stub ___ — 2005-06-05
• Stick ___ in the water — 2001-02-07
• Stub ___ — 1999-01-21