jats:pIn theoretical treatments of turbulent fountains, the entrainment of ambient fluid into the top of the fountain, hereinafter fountain-top entrainment jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline1" />jats:tex-math$Qtop$</jats:tex-math></jats:alternatives></jats:inline-formula> (jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline2" />jats:tex-math$m3 s-1$</jats:tex-math></jats:alternatives></jats:inline-formula>), has been neglected until now. This neglect, which modifies the energetic balance in a fountain, compromises the predictive ability of existing models. Our aim is to quantify jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline3" />jats:tex-math$Qtop$</jats:tex-math></jats:alternatives></jats:inline-formula> by shedding light on the physical processes that are responsible for fountain-top entrainment. First, estimates for jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline4" />jats:tex-math$Qtop$</jats:tex-math></jats:alternatives></jats:inline-formula> are obtained by applying, in turn, an entrainment closure in the vein of Morton jats:italicet al.</jats:italic> (jats:italicProc. R. Soc. Lond.</jats:italic>, vol. 234, 1956, pp. 1–23) and then of Shrinivas & Hunt (jats:italicJ. Fluid Mech.</jats:italic>, vol. 757, 2014, pp. 573–598) to the time-averaged fountain top. Unravelling the assumptions that underlie these approaches, we argue that neither capture the dynamical behaviour of the flow observed at the fountain top; the top being characterised by quasi-periodic fluctuations, during which large-scale eddies reverse and engulf parcels of ambient fluid into the fountain. Therefore, shifting our mindset to a periodical framework, we develop a new phenomenological model in which we emphasise the role of the fluctuations in entraining external fluid. Our model suggests that jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline5" />jats:tex-math$Qtop$</jats:tex-math></jats:alternatives></jats:inline-formula> is similar in magnitude to the volume flux supplied to the fountain top by the upflow (jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline6" />jats:tex-math$Qu$</jats:tex-math></jats:alternatives></jats:inline-formula>), i.e. jats:inline-formulajats:alternatives<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112016002330_inline7" />jats:tex-math$Qtop\sim Qu$</jats:tex-math></jats:alternatives></jats:inline-formula>, in agreement with experimental evidence. We conclude by providing guidance on how to implement fountain-top entrainment in existing models of turbulent fountains.</jats:p>