I have a layover in Houston on the way back from Mexico, and I have a small amount of money left in pesos. Will I be able to buy food with pesos in the airport, or would I have to exchange them first?
While you can exchange MXN into USD at almost any airport, you can get a much better exchange rate in Mexico. The best cash exchange rates are in Mexico City airport Terminal 1, where you can often find a spread as low as 1%.
No (buy food); yes (exchange them first).