To prepare the trick, simply place a ketchup packet inside a plastic bottle of water. Check that the ketchup packet floats in the water. Close the bottle tightly with the cap. 

Tell the audience that you will get the ketchup packet down with your mind power. However, as you can see in the video below, the ketchup packet doesn't move (because you're not doing anything). 

Next, involve the audience, ask them to help you. Ask them with the power of their mind to make the ketchup packet go down. Now squeeze the base of the bottle, without the audience realising, everyone will see the ketchup packet go down. Thank the audience for their support.

Note: Asking the audience for support is always a good idea, make them feel involved in the magic trick.

The ketchup packet has some air inside. Squeezing the bottle with water increases the pressure and the air in the ketchup packet is compressed. This compression leads to a decrease in the volume of the packet, thus an increase in its density, because the mass remains the same. The density of the ketchup packet becomes greater than the density of the water and it sinks.

When you let go of the bottle, the pressure decreases, returns to its initial value and the packet becomes less dense than the water in the bottle, causing it to float again.

Note: Sometimes the ketchup packet doesn't float, it sinks when placed in water. This is related to the density of the water used (which is not the same everywhere) and the density of the ketchup. Sometimes adjustments have to be made to the density of the water. If the ketchup sinks, we have to increase the density of the water, for example by adding salt.