... to drink from a nearby spring he discovered.
After all, it was a hot day, and he did feel thirsty.
He watched the water in the spring bubble and pour.
He grabbed his empty flask, and went up to the spring, and filled it up.
Seeing the water in the spring made him feel thirsty, so he foolishly took a sip from it.
Unbeknownst to Dave, the spring was actually a spring of transformation water, which would cause these changes to anyone foolish enough to drink from it:
Shrinking
Growing
Age regression
Tf
Dave, He put his flask into his Truck, got in, and drove off.