Annoying problems getting started with logic circuits (SN74LS)
The inputs of bipolar TTL (74nn, 74LSnn) parts will appear as a High when not connected, and will source 1 mA or so (varies with specific family) when taken low, so they are normally used with a switch between the input pin and Ground to ensure that the input will really be recognized as Low when we want it Low.  It is generally recommended to also use a 5 - 10K pull-up resistor from the input pin to Vcc, to ensure that the input will be seen as High when the switch is open.

CMOS parts (74HCnn, 74ACnn) have very high input impedances, and the inputs will float randomly if not connected.  All inputs (even unused ones) MUST have pull-up or pull down resistors to ensure that they will be in a known state when not driven High or Low by other means.