Water

Sorry! Your answer is wrong.
Outer air pressure causes the juice to stay in the glass.
To explain this, let´s have a closer look at the forces acting on the cardboard (see picture below):
1) The weight of the fluid acts on the cardboard from above. This pressure can be calculated by using the following formula:

p = ρ ⋅ g ⋅ h

When using a glass that is ten centimetres high the pressure adds up to:
p= 1000 kg/m ³ ⋅ 9,81 m/s² ⋅ 0,1 m = 981 N/m² (which is around 1000 N/m²)

2) Atmospheric pressure acts from below with a force of around 100 000 N/m² against the cardboard.

Since air pressure is one hundred times bigger than hydrostatic pressure, the cardboard is being pressed hard against the rim of the glass and the fluid can't escape.