Gravity
Because whatever you toss in the air will come back down, it's dangerous to throw heavy things. Tossing an apple or a ball in the air, for example, may be seen as a harmless activity. But because the object will pick up speed as it returns to earth, it could easily cause an injury if it lands on your head or elsewhere on your body.
Danger Overhead
A small, soft ball may not be seen as dangerous, but if you're throwing it up while inside a house, there are associated risks. In addition to harming yourself, the object could also hit a light, ceiling fan or hanging plant and cause damage. For this reason, tossing things up in the air is marginally safer outside, as there are rarely objects overhead.
Danger Below
While there are dangers above your head, there are also dangers below you that you may not consider while playing a game. Something thrown indoors could land on a glass table or potted plant, causing damage and a mess. Outdoors, throwing a ball near a garden, for example, could result in the object landing in the garden and harming a plant. Mischievous teenagers may find amusement in throwing rocks off an overpass, but when a rock hits a moving vehicle, significant damage or even death can occur.
Throwing Something Down
Though seemingly not as dangerous as throwing something up above your head, throwing something down can also have risks. If you throw something heavy, it could land on your foot, causing an injury, or similarly land on someone else or a pet. If what you throw is light, or bouncy, it could bounce off the ground and hit you on the way back up.