Take a pocket radio receiver for the FM band (88-108MHz). Fold the antenna in. Pick a radio station barely received. Set the volume loud.
Enclose the receiver in a plastic bag or several, make it water tight (Gefrierbeutel mit Verschluss?) (Good knot? Rubber bands?) (Tupperware?). Fasten the bag at mid-height of a bucket, which needs spacers and probably weights.
Fill the bucket with water. Tap water should suffice, but if not, take deionised water for car batteries. Let observe that the radio station is still heard (through water and so on, not obvious).
Dissolve de-icing salt in the water (or table salt for faster dissolution). Not a teaspoon: 250g/L, so take a true bag of salt. Let observe that the receiver doesn't catch the radio station any more.
If tap water has 1000Ω×cm=10Ω×m resistivity (this varies horribly!), then 0.2m×0.2m×0.2m have 50Ω. Free space impedance is 377Ω, and since λ/4~0.75m, the loss will be a bit stronger than 377/50≈18dB (the permittivity does a lot too). Most stations are still received. Deionised water would have no significant conductive loss.
25% NaCl in water conduct 22S/m or 45mΩ×m so 0.2m×0.2m×0.2m have 0.22Ω. Losses are now a bit over 64dB, so a station with properly chosen strength disappears.
By the way, this is what makes communications with submarines difficult.
This needs true preparation before showing it in front of students! Bag watertight, not floating, spacers stable, which radio station, hear the sound through water, water pure enough, time to dissolve the salt... The first trial won't work.