Conservation of Momentum Question

Hello,

Here is the situation I have been pondering:

A person is sitting in a shopping cart at rest. By throwing their weight forward they are able to cause the shopping cart with them inside to move. My question is how this works. If momentum is conserved unless an external force acts on the system and the initial momentum is zero when the cart is at rest, how is the shopping cart with the person inside able to move forward?

Thanks