Futures trade on a notional principal.
Let's take t-bonds for instance.
With $10k, you control $1M in LIBOR (London's equivalent to US treasury rate)
So on day 1, let's say you sell Apr futures are 97.1975 (market predicts LIBOR in Apr to be 2.0825%)
Let's say on day 2, futures are 97.2 (an increase of .0025)
Every day futures are marked to market.
So:
1,000,000 * .0025 = $25,000
You only had $10,000...but you must pay $25,000 to the clearinghouse.
So now you liquidate other assets to get out of the trade.