Re-Play Spill-Proof Sippy Cups

The best--only?--way to send a 1.5-6yo to bed with water and the confidence it won't leak. (These cups have little silicone things in the spout, so they only release water with the pressure of drinking.) One of the only specialty kid items we always travel with, as they can be used anywhere a cup is needed--for milk, in the bath, etc.