This is a joint talk with PACM/MAE.   To tie a shoelace into a knot is a relatively simple affair. Tying a knot in a field is a different story, because the whole of space must be filled in a way that matches the knot being tied at the core. The possibility of such localized knottedness in a space-filling field has fascinated physicists and mathematicians ever since Kelvin’s 'vortex atom' hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types.  More recently, knottiness (Helicity) has re-emerged as a  conserved quantity in manyidealized situations (such as Euler fluids and ideal plasmas). By presenting experiments, simulations and analytical work, I will talk about how this topological quantity can be more or less conserved than you might have expected in light, fluids and superfluids